diff --git a/lib/config/validator.ts b/lib/config/validator.ts
index 2f0dd041..23084361 100644
--- a/lib/config/validator.ts
+++ b/lib/config/validator.ts
@@ -54,6 +54,18 @@ export default function constructAndValidateConfiguration(
     // Required fields
     if (!repositories || !repositories.length) {
         errors.push(`Missing repositories.`);
+    } else {
+        const duplicateRepositories = repositories.filter(
+            (item, index, array) => array.indexOf(item) !== index
+        );
+
+        if (duplicateRepositories.length) {
+            errors.push(
+                `repositories contains duplicate entries: [${duplicateRepositories.join(
+                    ', '
+                )}]`
+            );
+        }
     }
 
     if (!extensions || !extensions.length) {
diff --git a/test/validator.test.ts b/test/validator.test.ts
index c84955af..d75a6fc6 100644
--- a/test/validator.test.ts
+++ b/test/validator.test.ts
@@ -3,7 +3,7 @@ import validator from '@config/validator';
 import { ConfigToValidate } from '@config/types';
 
 const DEFAULT_CONFIGURATION: ConfigToValidate = {
-    repositories: ['test-repo'],
+    repositories: ['test-repo', 'test-repo-2'],
     extensions: ['.ts', '.tsx'],
     pathIgnorePattern: undefined,
     maxFileSizeBytes: undefined,
@@ -17,6 +17,13 @@ const DEFAULT_CONFIGURATION: ConfigToValidate = {
 };
 
 describe('Config validator', () => {
+    test('valid configuration is accepted', () => {
+        validator(DEFAULT_CONFIGURATION);
+
+        const validationErrors = getConsoleLogCalls();
+        expect(validationErrors.join(',')).toBeFalsy();
+    });
+
     test('repositories are required', () => {
         const config = { ...DEFAULT_CONFIGURATION, repositories: undefined! };
         validator(config);
@@ -26,6 +33,25 @@ describe('Config validator', () => {
         expect(validationError).toMatch('- Missing repositories.');
     });
 
+    test('duplicate repositories are unsupported', () => {
+        const repositories = [
+            'duplicate-1',
+            'valid',
+            'duplicate-1',
+            'duplicate-2',
+            'valid-2',
+            'duplicate-2',
+        ];
+        const config = { ...DEFAULT_CONFIGURATION, repositories };
+        validator(config);
+
+        const [validationError] = getConsoleLogCalls();
+        expect(validationError).toMatch('Configuration validation errors:');
+        expect(validationError).toMatch(
+            '- repositories contains duplicate entries: [duplicate-1, duplicate-2]'
+        );
+    });
+
     test('additional options are unsupported', () => {
         const key = 'someMistypedKey';
         const config = { ...DEFAULT_CONFIGURATION, [key]: true };