diff --git a/src/language/validation/other/types/namedTypes.ts b/src/language/validation/other/types/namedTypes.ts
new file mode 100644
index 000000000..e53e6967e
--- /dev/null
+++ b/src/language/validation/other/types/namedTypes.ts
@@ -0,0 +1,48 @@
+import { SdsNamedType } from '../../../generated/ast.js';
+import { ValidationAcceptor } from 'langium';
+import { SafeDsServices } from '../../../safe-ds-module.js';
+import { typeArgumentsOrEmpty } from '../../../helpers/nodeProperties.js';
+import { duplicatesBy } from '../../../helpers/collectionUtils.js';
+
+export const CODE_NAMED_TYPE_DUPLICATE_TYPE_PARAMETER = 'named-type/duplicate-type-parameter';
+export const CODE_NAMED_TYPE_POSITIONAL_AFTER_NAMED = 'named-type/positional-after-named';
+
+export const namedTypeMustNotSetTypeParameterMultipleTimes = (services: SafeDsServices) => {
+    const nodeMapper = services.helpers.NodeMapper;
+    const typeArgumentToTypeParameterOrUndefined = nodeMapper.typeArgumentToTypeParameterOrUndefined.bind(nodeMapper);
+
+    return (node: SdsNamedType, accept: ValidationAcceptor): void => {
+        const typeArguments = typeArgumentsOrEmpty(node.typeArgumentList);
+        const duplicates = duplicatesBy(typeArguments, typeArgumentToTypeParameterOrUndefined);
+
+        for (const duplicate of duplicates) {
+            const correspondingTypeParameter = typeArgumentToTypeParameterOrUndefined(duplicate)!;
+            accept('error', `The type parameter '${correspondingTypeParameter.name}' is already set.`, {
+                node: duplicate,
+                code: CODE_NAMED_TYPE_DUPLICATE_TYPE_PARAMETER,
+            });
+        }
+    };
+};
+
+export const namedTypeTypeArgumentListMustNotHavePositionalArgumentsAfterNamedArguments = (
+    node: SdsNamedType,
+    accept: ValidationAcceptor,
+): void => {
+    const typeArgumentList = node.typeArgumentList;
+    if (!typeArgumentList) {
+        return;
+    }
+
+    let foundNamed = false;
+    for (const typeArgument of typeArgumentList.typeArguments) {
+        if (typeArgument.typeParameter) {
+            foundNamed = true;
+        } else if (foundNamed) {
+            accept('error', 'After the first named type argument all type arguments must be named.', {
+                node: typeArgument,
+                code: CODE_NAMED_TYPE_POSITIONAL_AFTER_NAMED,
+            });
+        }
+    }
+};
diff --git a/src/language/validation/other/types/typeArgumentLists.ts b/src/language/validation/other/types/typeArgumentLists.ts
deleted file mode 100644
index aae56f5ce..000000000
--- a/src/language/validation/other/types/typeArgumentLists.ts
+++ /dev/null
@@ -1,21 +0,0 @@
-import { SdsTypeArgumentList } from '../../../generated/ast.js';
-import { ValidationAcceptor } from 'langium';
-
-export const CODE_TYPE_ARGUMENT_LIST_POSITIONAL_AFTER_NAMED = 'type-argument-list/positional-after-named';
-
-export const typeArgumentListMustNotHavePositionalArgumentsAfterNamedArguments = (
-    node: SdsTypeArgumentList,
-    accept: ValidationAcceptor,
-): void => {
-    let foundNamed = false;
-    for (const typeArgument of node.typeArguments) {
-        if (typeArgument.typeParameter) {
-            foundNamed = true;
-        } else if (foundNamed) {
-            accept('error', 'After the first named type argument all type arguments must be named.', {
-                node: typeArgument,
-                code: CODE_TYPE_ARGUMENT_LIST_POSITIONAL_AFTER_NAMED,
-            });
-        }
-    }
-};
diff --git a/src/language/validation/safe-ds-validator.ts b/src/language/validation/safe-ds-validator.ts
index c3b00059f..7c2b9a396 100644
--- a/src/language/validation/safe-ds-validator.ts
+++ b/src/language/validation/safe-ds-validator.ts
@@ -48,7 +48,6 @@ import {
     callableTypeMustNotHaveOptionalParameters,
     callableTypeParameterMustNotHaveConstModifier,
 } from './other/types/callableTypes.js';
-import { typeArgumentListMustNotHavePositionalArgumentsAfterNamedArguments } from './other/types/typeArgumentLists.js';
 import { argumentListMustNotHavePositionalArgumentsAfterNamedArguments } from './other/argumentLists.js';
 import {
     referenceMustNotBeFunctionPointer,
@@ -82,6 +81,10 @@ import {
 import { memberAccessMustBeNullSafeIfReceiverIsNullable } from './other/expressions/memberAccesses.js';
 import { importPackageMustExist, importPackageShouldNotBeEmpty } from './other/imports.js';
 import { singleUseAnnotationsMustNotBeRepeated } from './builtins/repeatable.js';
+import {
+    namedTypeMustNotSetTypeParameterMultipleTimes,
+    namedTypeTypeArgumentListMustNotHavePositionalArgumentsAfterNamedArguments
+} from "./other/types/namedTypes.js";
 
 /**
  * Register custom validation checks.
@@ -140,8 +143,10 @@ export const registerValidationChecks = function (services: SafeDsServices) {
         SdsNamedType: [
             namedTypeDeclarationShouldNotBeDeprecated(services),
             namedTypeDeclarationShouldNotBeExperimental(services),
+            namedTypeMustNotSetTypeParameterMultipleTimes(services),
             namedTypeMustSetAllTypeParameters(services),
             namedTypeTypeArgumentListShouldBeNeeded,
+            namedTypeTypeArgumentListMustNotHavePositionalArgumentsAfterNamedArguments,
         ],
         SdsParameter: [
             parameterMustHaveTypeHint,
@@ -165,7 +170,6 @@ export const registerValidationChecks = function (services: SafeDsServices) {
             segmentResultListShouldNotBeEmpty,
         ],
         SdsTemplateString: [templateStringMustHaveExpressionBetweenTwoStringParts],
-        SdsTypeArgumentList: [typeArgumentListMustNotHavePositionalArgumentsAfterNamedArguments],
         SdsTypeParameterConstraint: [typeParameterConstraintLeftOperandMustBeOwnTypeParameter],
         SdsTypeParameterList: [typeParameterListShouldNotBeEmpty],
         SdsUnionType: [unionTypeMustHaveTypeArguments, unionTypeShouldNotHaveASingularTypeArgument],
diff --git a/tests/resources/validation/other/types/type argument lists/duplicate type parameter/main.sdstest b/tests/resources/validation/other/types/type argument lists/duplicate type parameter/main.sdstest
new file mode 100644
index 000000000..0cbf53fec
--- /dev/null
+++ b/tests/resources/validation/other/types/type argument lists/duplicate type parameter/main.sdstest	
@@ -0,0 +1,36 @@
+package tests.validation.other.types.typeArgumentLists.duplicateTypeParameters
+
+class MyClass<A, B>
+
+fun myFunction(
+    f: MyClass<
+        // $TEST$ no error r"The type parameter '\w+' is already set\."
+        »Int«,
+        // $TEST$ error "The type parameter 'A' is already set."
+        »A = Int«
+    >,
+    g: MyClass<
+        // $TEST$ no error r"The type parameter '\w+' is already set\."
+        »B = Int«,
+        // $TEST$ error "The type parameter 'B' is already set."
+        »B = Int«
+    >,
+    h: MyClass<
+        // $TEST$ no error r"The type parameter '\w+' is already set\."
+        »A = Int«,
+        // $TEST$ no error r"The type parameter '\w+' is already set\."
+        »B = Int«
+    >,
+    i: MyClass<
+        // $TEST$ no error r"The type parameter '\w+' is already set\."
+        »Int«,
+        // $TEST$ no error r"The type parameter '\w+' is already set\."
+        »Int«
+    >,
+    j: MyClass<
+        // $TEST$ no error r"The type parameter '\w+' is already set\."
+        »Unresolved = Int«,
+        // $TEST$ no error r"The type parameter '\w+' is already set\."
+        »Unresolved = Int«
+    >
+)