diff --git a/backend/src/config/auth.config.ts b/backend/src/config/auth.config.ts
index 4e91965f1..e561f627b 100644
--- a/backend/src/config/auth.config.ts
+++ b/backend/src/config/auth.config.ts
@@ -10,6 +10,7 @@ import * as Joi from 'joi';
 import { GitlabScope, GitlabVersion } from './gitlab.enum';
 import {
   buildErrorMessage,
+  ensureNoDuplicatesExist,
   parseOptionalNumber,
   replaceAuthErrorsWithEnvironmentVariables,
   toArrayConfig,
@@ -217,7 +218,6 @@ const authSchema = Joi.object({
 });
 
 export default registerAs('authConfig', () => {
-  // ToDo: Validate these with Joi to prevent duplicate entries?
   const gitlabNames = (
     toArrayConfig(process.env.HD_AUTH_GITLABS, ',') ?? []
   ).map((name) => name.toUpperCase());
@@ -226,9 +226,13 @@ export default registerAs('authConfig', () => {
       "GitLab auth is currently not yet supported. Please don't configure it",
     );
   }
+  ensureNoDuplicatesExist('GitLab', gitlabNames);
+
   const ldapNames = (
     toArrayConfig(process.env.HD_AUTH_LDAP_SERVERS, ',') ?? []
   ).map((name) => name.toUpperCase());
+  ensureNoDuplicatesExist('LDAP', ldapNames);
+
   const samlNames = (toArrayConfig(process.env.HD_AUTH_SAMLS, ',') ?? []).map(
     (name) => name.toUpperCase(),
   );
@@ -237,6 +241,8 @@ export default registerAs('authConfig', () => {
       "SAML auth is currently not yet supported. Please don't configure it",
     );
   }
+  ensureNoDuplicatesExist('SAML', samlNames);
+
   const oauth2Names = (
     toArrayConfig(process.env.HD_AUTH_OAUTH2S, ',') ?? []
   ).map((name) => name.toUpperCase());
@@ -245,6 +251,7 @@ export default registerAs('authConfig', () => {
       "OAuth2 auth is currently not yet supported. Please don't configure it",
     );
   }
+  ensureNoDuplicatesExist('OAuth2', oauth2Names);
 
   const gitlabs = gitlabNames.map((gitlabName) => {
     return {
diff --git a/backend/src/config/utils.spec.ts b/backend/src/config/utils.spec.ts
index 6249611cc..62a57b030 100644
--- a/backend/src/config/utils.spec.ts
+++ b/backend/src/config/utils.spec.ts
@@ -1,10 +1,12 @@
 /*
- * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ * SPDX-FileCopyrightText: 2023 The HedgeDoc developers (see AUTHORS file)
  *
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 import { Loglevel } from './loglevel.enum';
 import {
+  ensureNoDuplicatesExist,
+  findDuplicatesInArray,
   needToLog,
   parseOptionalNumber,
   replaceAuthErrorsWithEnvironmentVariables,
@@ -12,6 +14,39 @@ import {
 } from './utils';
 
 describe('config utils', () => {
+  describe('findDuplicatesInArray', () => {
+    it('empty array', () => {
+      expect(findDuplicatesInArray([])).toEqual([]);
+    });
+    it('returns empty array when input does not have any duplicates', () => {
+      expect(findDuplicatesInArray(['A', 'B'])).toEqual([]);
+    });
+    it('returns duplicates if input contains a duplicate', () => {
+      expect(findDuplicatesInArray(['A', 'B', 'A'])).toEqual(['A']);
+    });
+    it('returns duplicates if input contains a duplicate twice', () => {
+      expect(findDuplicatesInArray(['A', 'B', 'A', 'A'])).toEqual(['A']);
+    });
+    it('returns duplicates if input contains multiple duplicates', () => {
+      expect(findDuplicatesInArray(['A', 'B', 'A', 'B'])).toEqual(['A', 'B']);
+    });
+  });
+  describe('ensureNoDuplicatesExist', () => {
+    // eslint-disable-next-line jest/expect-expect
+    it('throws no error if everything is correct', () => {
+      ensureNoDuplicatesExist('Test', ['A']);
+    });
+    it('throws error if there is a duplicate', () => {
+      expect(() => ensureNoDuplicatesExist('Test', ['A', 'A'])).toThrow(
+        "Your Test names 'A,A' contain duplicates 'A'",
+      );
+    });
+    it('throws error if there are multiple duplicates', () => {
+      expect(() =>
+        ensureNoDuplicatesExist('Test', ['A', 'A', 'B', 'B']),
+      ).toThrow("Your Test names 'A,A,B,B' contain duplicates 'A,B'");
+    });
+  });
   describe('toArrayConfig', () => {
     it('empty', () => {
       expect(toArrayConfig('')).toEqual(undefined);
diff --git a/backend/src/config/utils.ts b/backend/src/config/utils.ts
index f068f3d1d..64d897094 100644
--- a/backend/src/config/utils.ts
+++ b/backend/src/config/utils.ts
@@ -1,10 +1,31 @@
 /*
- * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ * SPDX-FileCopyrightText: 2023 The HedgeDoc developers (see AUTHORS file)
  *
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 import { Loglevel } from './loglevel.enum';
 
+export function findDuplicatesInArray<T>(array: T[]): T[] {
+  // This uses the Array-Set conversion to remove duplicates in the finding.
+  // This can happen if an entry is present three or more times
+  return Array.from(
+    new Set(array.filter((item, index) => array.indexOf(item) !== index)),
+  );
+}
+
+export function ensureNoDuplicatesExist(
+  authName: string,
+  names: string[],
+): void {
+  const duplicates = findDuplicatesInArray(names);
+  if (duplicates.length !== 0) {
+    throw new Error(
+      `Your ${authName} names '${names.join(
+        ',',
+      )}' contain duplicates '${duplicates.join(',')}'`,
+    );
+  }
+}
 export function toArrayConfig(
   configValue?: string,
   separator = ',',