diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5587af1..239e526 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -38,6 +38,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A redefinition of an `attributeGroup` that SELF-references the original and also directly re-declares one of its attributes is now rejected. The self-reference re-imports all of the original's attribute uses, so a same-named direct re-declaration produces two attribute uses of the same name, a duplicate forbidden by ag-props-correct.2. The check fires only for a re-declared, non-prohibited attribute the original directly declares, and only when both the original and the re-declaration land in no namespace (a qualified attribute's target namespace depends on cross-document `form`/chameleon resolution, so it is not compared, staying lenient); a self-referencing redefinition that adds genuinely new attributes (attgC007/attgC038/groupB018) stays valid. Catches schM8 (`c-g2` self-references and re-declares `a1`). XSTS invalid-schemas-accepted 23 → 22 (schM8), valid-schemas-rejected held at 0.
 - A redefinition of an `attributeGroup` that WIDENS a re-declared attribute's type is now rejected. A redefinition is a restriction (src-redefine / cos-ct-restricts), so a re-declared attribute's type must derive by restriction from the original's. The check is built-in-only and therefore certain: it fires only when both the original and the re-declared attribute name a built-in type (resolved to the XSD namespace) and the re-declared built-in does not derive from the original's in the XSD derivation lattice (`int` does not derive from `boolean`). A user, inline, or absent type on either side is not compared (declines, staying lenient): an absent original type is `anySimpleType`, which any type validly restricts, a user/inline type's derivation is not resolvable here, and a user type whose local name coincides with a built-in (a `{urn:x}int`) resolves to a different namespace and is not mistaken for the built-in, so no valid restriction is rejected. Catches schM4 (`b1` re-typed `boolean` → `int`). XSTS invalid-schemas-accepted 24 → 23 (schM4), valid-schemas-rejected held at 0.
 - A redefinition of an `attributeGroup` that adds an attribute the original lacks through a NON-self `attributeGroup` reference is now rejected. A redefinition is a restriction of the original (src-redefine / cos-ct-restricts), so it may not introduce an attribute the original does not declare unless the original carries an attribute wildcard. The add-attribute check previously resolved only the redefinition's DIRECT declarations and was suppressed whenever the redefinition carried any `attributeGroup` reference, which conflated a self-reference (re-importing the original's own attributes, valid) with a non-self reference (injecting foreign attributes, invalid). The redefinition's contributed attribute names are now resolved through its non-self references and the suppression keys on a genuine self-reference (matched by namespace and local name); resolution declines, staying lenient, whenever the contributed set is not fully knowable (an unresolved group, an attribute-by-`ref`, an `anyAttribute` wildcard, or a further nested reference). This catches attgC028 (a redefinition whose `ref="car"` replaces the original's attributes with foreign ones) while keeping attgC007/attgC038/groupB018 (added attributes alongside a self-reference) valid. XSTS invalid-schemas-accepted 25 → 24 (attgC028), valid-schemas-rejected held at 0.
 - An unprefixed `type`/`base`/`itemType`/`memberTypes` reference to a type declared in a non-empty target namespace is now rejected as undeclared. With no default `xmlns`, an unprefixed QName resolves to the absent namespace, so `type="t"` is `{}t`, not `{urn:x}t` where the type lives; `isUndeclaredReferenceType` matched the type by local name and accepted it. It now mirrors `isUndeclaredReferenceRef`: a no-namespace reference is undeclared when the schema has a non-empty target namespace (a chameleon-included global, and an imported no-namespace type via the foreign pools, remain accepted). XSTS invalid-schemas-accepted 27 → 25 (addB009/test66059 and one more), valid-schemas-rejected held at 0.
diff --git a/Sources/Schema/SchemaRedefineSelfReference.swift b/Sources/Schema/SchemaRedefineSelfReference.swift
index 0778458..9a78d60 100644
--- a/Sources/Schema/SchemaRedefineSelfReference.swift
+++ b/Sources/Schema/SchemaRedefineSelfReference.swift
@@ -113,15 +113,12 @@ extension PureXML.Schema.XSDParser {
     }
 
     /// The non-prohibited attribute names a redefinition pulls in through its
-    /// `attributeGroup` references, with whether any of them is a SELF-reference
-    /// (resolving by NAMESPACE and local name to the redefinition's own name+target).
-    /// A self-reference re-imports the original and is reported via `hasSelfReference`
-    /// rather than counted. Returns nil (decline, stay lenient) when a referenced
-    /// group does not resolve from the base pool, carries an attribute wildcard, nests
-    /// a further `attributeGroup` reference, or names an attribute by `ref` (not
-    /// `name`): in each case the contributed set is not fully known, so the add-check
-    /// must not fire. A redefinition with no `attributeGroup` reference yields an empty
-    /// set and no self-reference, preserving the direct-declaration-only behavior.
+    /// `attributeGroup` references, with whether any is a SELF-reference (resolving by
+    /// NAMESPACE and local name to the redefinition's own name+target; reported via
+    /// `hasSelfReference` rather than counted, since it re-imports the original). Returns
+    /// nil (decline, stay lenient) when the contributed set is not fully known: a
+    /// referenced group that does not resolve, carries an attribute wildcard, nests a
+    /// further `attributeGroup` reference, or names an attribute by `ref` (not `name`).
     private static func referencedAttributeNames(
         of redefinition: XSDTree, ownName: String, base: [String: XSDTree],
     ) -> (names: Set<String>, hasSelfReference: Bool)? {
@@ -197,13 +194,16 @@ extension PureXML.Schema.XSDParser {
             if use(attrName) != "prohibited", widensBuiltinType(base: attribute, redefined: redefined[attrName]) {
                 errors.append("a redefined attribute group '\(name)' may not widen the type of '\(attrName)'")
             }
-            switch RedefineNode.attribute(attribute, "use") {
-            case "required" where !hasReference && redefined[attrName] == nil:
-                errors.append("a redefined attribute group '\(name)' may not eliminate the required attribute '\(attrName)'")
-            case "prohibited" where !baseHasWildcard && use(attrName) != nil && use(attrName) != "prohibited":
-                errors.append("a redefined attribute group '\(name)' may not re-introduce the prohibited attribute '\(attrName)'")
-            default:
-                break
+            if redeclaresSelfSuppliedAttribute(
+                attribute,
+                redeclaration: redefined[attrName],
+                hasSelfReference: referenced?.hasSelfReference == true,
+                prohibited: use(attrName) == "prohibited",
+            ) {
+                errors.append("a redefined attribute group '\(name)' redeclares '\(attrName)', which its self-reference already supplies")
+            }
+            if let transition = useTransitionError(base: attribute, redefined: redefined[attrName], name: name, hasReference: hasReference, baseHasWildcard: baseHasWildcard) {
+                errors.append(transition)
             }
         }
         return errors
@@ -233,6 +233,49 @@ extension PureXML.Schema.XSDParser {
             .map { "a redefined attribute group '\(name)' may not add the attribute '\($0)'" }
     }
 
+    /// The `use`-transition restriction errors for one base attribute: a redefinition
+    /// may not eliminate a REQUIRED attribute (unless a reference could re-supply it),
+    /// nor re-introduce a PROHIBITED attribute as usable (unless the original has an
+    /// attribute wildcard that could admit it).
+    private static func useTransitionError(
+        base: XSDTree, redefined: XSDTree?, name: String, hasReference: Bool, baseHasWildcard: Bool,
+    ) -> String? {
+        guard let attrName = RedefineNode.attribute(base, "name") else { return nil }
+        let use = redefined.map { RedefineNode.attribute($0, "use") ?? "optional" }
+        switch RedefineNode.attribute(base, "use") {
+        case "required" where !hasReference && redefined == nil:
+            return "a redefined attribute group '\(name)' may not eliminate the required attribute '\(attrName)'"
+        case "prohibited" where !baseHasWildcard && use != nil && use != "prohibited":
+            return "a redefined attribute group '\(name)' may not re-introduce the prohibited attribute '\(attrName)'"
+        default:
+            return nil
+        }
+    }
+
+    /// schM8 / ag-props-correct.2: when a redefinition SELF-references the original it
+    /// re-imports all of the original's attributes, so directly re-declaring one of them
+    /// produces two attribute uses of the same name (a duplicate), which is invalid. The
+    /// check fires only for a re-declared, NON-prohibited attribute the original directly
+    /// declares, and only when BOTH the original and the re-declaration land in NO
+    /// namespace (a qualified attribute's target namespace depends on cross-document
+    /// `form`/chameleon resolution, so it is not compared, staying lenient).
+    private static func redeclaresSelfSuppliedAttribute(
+        _ attribute: XSDTree, redeclaration: XSDTree?, hasSelfReference: Bool, prohibited: Bool,
+    ) -> Bool {
+        guard hasSelfReference, let redeclaration, !prohibited else { return false }
+        return isNoNamespaceAttribute(attribute) && isNoNamespaceAttribute(redeclaration)
+    }
+
+    /// Whether an attribute use lands in NO namespace: it is not `form="qualified"` and
+    /// its owning schema does not set `attributeFormDefault="qualified"`. A qualified
+    /// attribute's target namespace depends on cross-document `form`/chameleon
+    /// resolution, so the self-reference duplicate check (which compares by local name)
+    /// is sound only for no-namespace attributes; a qualified one is not compared.
+    private static func isNoNamespaceAttribute(_ attribute: XSDTree) -> Bool {
+        if RedefineNode.attribute(attribute, "form") == "qualified" { return false }
+        return RedefineNode.attribute(RedefineNode.schemaOwner(attribute), "attributeFormDefault") != "qualified"
+    }
+
     /// Whether a redefined attribute WIDENS the original's type. A redefinition is a
     /// restriction, so a re-declared attribute's type must derive by restriction from
     /// the original's. The check is BUILT-IN-only and therefore certain: it fires only
diff --git a/Tests/SchemaRedefineAttributeGroupTests.swift b/Tests/SchemaRedefineAttributeGroupTests.swift
index 02800f0..0695c42 100644
--- a/Tests/SchemaRedefineAttributeGroupTests.swift
+++ b/Tests/SchemaRedefineAttributeGroupTests.swift
@@ -80,6 +80,28 @@ struct SchemaRedefineAttributeGroupTests {
         )
     }
 
+    @Test("re-declaring an attribute the self-reference already supplies is rejected (schM8)")
+    func test_selfRefRedeclareDuplicateRejected() throws {
+        // The self-reference re-imports the original's `a`, so directly re-declaring `a`
+        // produces two attribute uses of the same name: a duplicate (ag-props-correct.2).
+        #expect(throws: (any Error).self) {
+            try redefine(
+                group: "<xs:attributeGroup ref='g'/><xs:attribute name='a' type='xs:string'/>",
+                base: "<xs:attribute name='a' type='xs:string'/>",
+            )
+        }
+    }
+
+    @Test("a self-reference re-declaring a prohibited attribute is not flagged as duplicate (lenient)")
+    func test_selfRefRedeclareProhibitedLenient() throws {
+        // A `use="prohibited"` re-declaration removes rather than duplicates, so the
+        // duplicate check declines; the base attribute is optional, so this compiles.
+        _ = try redefine(
+            group: "<xs:attributeGroup ref='g'/><xs:attribute name='a' use='prohibited' type='xs:string'/>",
+            base: "<xs:attribute name='a' type='xs:string'/>",
+        )
+    }
+
     @Test("adding an attribute alongside a self-reference is valid")
     func test_addWithSelfReferenceAccepted() throws {
         _ = try redefine(
diff --git a/Tests/XSTSSuiteTests.swift b/Tests/XSTSSuiteTests.swift
index 71af5c9..51d922c 100644
--- a/Tests/XSTSSuiteTests.swift
+++ b/Tests/XSTSSuiteTests.swift
@@ -146,9 +146,9 @@ struct XSTSSuiteTests {
     /// model-group occurrence, identity-constraint content, derivation base kind, attribute/simple-type,
     /// composition, simpleContent inline-type, complexContent mixedness, and substitution-group anySimpleType checks.
     /// src-resolve (#185) 43->39; xsi:Not-Allowed (#183) 39->37; NameAndTypeOK restriction-only + cos-all-limited group-ref (#158) 37->35.
-    /// UPA `##other` 31; ext-model 29; XSD-ns-ref 27; unprefixed-type-ref 25; redefine non-self-add attgC028 24; redefine type-widen schM4 23; valid-rejected 0.
+    /// UPA `##other` 31; ext-model 29; XSD-ns-ref 27; unprefixed-type-ref 25; redefine non-self-add attgC028 24; type-widen schM4 23; self-ref dup schM8 22; valid-rejected 0.
     private let knownSchemaValidRejected = 0
-    private let knownSchemaInvalidAccepted = 23
+    private let knownSchemaInvalidAccepted = 22
     private let knownInstanceValidRejected = 0
     /// Instance bucket (133 -> 26), per-step deltas in CHANGELOG.md: xsi:type must derive from the
     /// declared type (complex/atomic/list/union via the element-ref chain); anyType cannot stand in