diff --git a/packages/blocks/src/store/reducer.js b/packages/blocks/src/store/reducer.js
index c00810c534d55d..ed7b4e74b0998c 100644
--- a/packages/blocks/src/store/reducer.js
+++ b/packages/blocks/src/store/reducer.js
@@ -328,7 +328,12 @@ export const groupingBlockName = createBlockNameSetterReducer(
 export function categories( state = DEFAULT_CATEGORIES, action ) {
 	switch ( action.type ) {
 		case 'SET_CATEGORIES':
-			return action.categories || [];
+			// Ensure, that categories are unique by slug.
+			const uniqueCategories = new Map();
+			( action.categories || [] ).forEach( ( category ) => {
+				uniqueCategories.set( category.slug, category );
+			} );
+			return [ ...uniqueCategories.values() ];
 		case 'UPDATE_CATEGORY': {
 			if (
 				! action.category ||
diff --git a/packages/blocks/src/store/test/reducer.js b/packages/blocks/src/store/test/reducer.js
index babaaad4e0e0d7..1a295f1efc30d7 100644
--- a/packages/blocks/src/store/test/reducer.js
+++ b/packages/blocks/src/store/test/reducer.js
@@ -420,6 +420,22 @@ describe( 'categories', () => {
 		expect( state ).toEqual( [ { slug: 'wings', title: 'Wings' } ] );
 	} );
 
+	it( 'should ensure, that categories are unique by slug', () => {
+		const original = deepFreeze( [
+			{ slug: 'chicken', title: 'Chicken' },
+		] );
+
+		const state = categories( original, {
+			type: 'SET_CATEGORIES',
+			categories: [ { slug: 'chicken', title: 'Another chicken' } ],
+		} );
+
+		expect( state ).toEqual( [
+			{ slug: 'chicken', title: 'Another chicken' },
+		] );
+		expect( state.length ).toBe( 1 );
+	} );
+
 	it( 'should add the category icon', () => {
 		const original = deepFreeze( [
 			{