diff --git a/lib/productions/interface.js b/lib/productions/interface.js
index 202bf947..2abf7212 100644
--- a/lib/productions/interface.js
+++ b/lib/productions/interface.js
@@ -5,6 +5,7 @@ import { Constant } from "./constant.js";
 import { IterableLike } from "./iterable.js";
 import { stringifier } from "./helpers.js";
 import { validationError } from "../error.js";
+import { checkInterfaceMemberDuplication } from "../validators/interface.js";
 
 /**
  * @param {import("../tokeniser").Tokeniser} tokeniser
@@ -52,5 +53,8 @@ for more information.`;
       yield validationError(this.source, this.tokens.name, this, message);
     }
     yield* super.validate(defs);
+    if (!this.partial) {
+      yield* checkInterfaceMemberDuplication(defs, this);
+    }
   }
 }
diff --git a/lib/validator.js b/lib/validator.js
index e247ae45..93baeb12 100644
--- a/lib/validator.js
+++ b/lib/validator.js
@@ -2,6 +2,24 @@
 
 import { validationError as error } from "./error.js";
 
+function getMixinMap(all, unique) {
+  const map = new Map();
+  const includes = all.filter(def => def.type === "includes");
+  for (const include of includes) {
+    const mixin = unique.get(include.includes);
+    if (!mixin) {
+      continue;
+    }
+    const array = map.get(include.target);
+    if (array) {
+      array.push(mixin);
+    } else {
+      map.set(include.target, [mixin]);
+    }
+  }
+  return map;
+}
+
 function groupDefinitions(all) {
   const unique = new Map();
   const duplicates = new Set();
@@ -25,7 +43,13 @@ function groupDefinitions(all) {
       duplicates.add(def);
     }
   }
-  return { all, unique, partials, duplicates };
+  return {
+    all,
+    unique,
+    partials,
+    duplicates,
+    mixinMap: getMixinMap(all, unique)
+  };
 }
 
 function* checkDuplicatedNames({ unique, duplicates }) {
@@ -36,61 +60,6 @@ function* checkDuplicatedNames({ unique, duplicates }) {
   }
 }
 
-function* checkInterfaceMemberDuplication(defs) {
-  const interfaces = [...defs.unique.values()].filter(def => def.type === "interface");
-  const includesMap = getIncludesMap();
-
-  for (const i of interfaces) {
-    yield* forEachInterface(i);
-  }
-
-  function* forEachInterface(i) {
-    const opNames = new Set(getOperations(i).map(op => op.name));
-    const partials = defs.partials.get(i.name) || [];
-    const mixins = includesMap.get(i.name) || [];
-    for (const ext of [...partials, ...mixins]) {
-      const additions = getOperations(ext);
-      yield* forEachExtension(additions, opNames, ext, i);
-      for (const addition of additions) {
-        opNames.add(addition.name);
-      }
-    }
-  }
-
-  function* forEachExtension(additions, existings, ext, base) {
-    for (const addition of additions) {
-      const { name } = addition;
-      if (name && existings.has(name)) {
-        const message = `The operation "${name}" has already been defined for the base interface "${base.name}" either in itself or in a mixin`;
-        yield error(ext.source, addition.tokens.name, ext, message);
-      }
-    }
-  }
-
-  function getOperations(i) {
-    return i.members
-      .filter(({type}) => type === "operation");
-  }
-
-  function getIncludesMap() {
-    const map = new Map();
-    const includes = defs.all.filter(def => def.type === "includes");
-    for (const include of includes) {
-      const array = map.get(include.target);
-      const mixin = defs.unique.get(include.includes);
-      if (!mixin) {
-        continue;
-      }
-      if (array) {
-        array.push(mixin);
-      } else {
-        map.set(include.target, [mixin]);
-      }
-    }
-    return map;
-  }
-}
-
 function* validateIterable(ast) {
   const defs = groupDefinitions(ast);
   for (const def of defs.all) {
@@ -99,7 +68,6 @@ function* validateIterable(ast) {
     }
   }
   yield* checkDuplicatedNames(defs);
-  yield* checkInterfaceMemberDuplication(defs);
 }
 
 // Remove this once all of our support targets expose `.flat()` by default
diff --git a/lib/validators/interface.js b/lib/validators/interface.js
new file mode 100644
index 00000000..c71fe706
--- /dev/null
+++ b/lib/validators/interface.js
@@ -0,0 +1,29 @@
+import { validationError } from "../error.js";
+
+export function* checkInterfaceMemberDuplication(defs, i) {
+  const opNames = new Set(getOperations(i).map(op => op.name));
+  const partials = defs.partials.get(i.name) || [];
+  const mixins = defs.mixinMap.get(i.name) || [];
+  for (const ext of [...partials, ...mixins]) {
+    const additions = getOperations(ext);
+    yield* forEachExtension(additions, opNames, ext, i);
+    for (const addition of additions) {
+      opNames.add(addition.name);
+    }
+  }
+
+  function* forEachExtension(additions, existings, ext, base) {
+    for (const addition of additions) {
+      const { name } = addition;
+      if (name && existings.has(name)) {
+        const message = `The operation "${name}" has already been defined for the base interface "${base.name}" either in itself or in a mixin`;
+        yield validationError(ext.source, addition.tokens.name, ext, message);
+      }
+    }
+  }
+
+  function getOperations(i) {
+    return i.members
+      .filter(({type}) => type === "operation");
+  }
+}