diff --git a/src/api/endpoints/posts/create.js b/src/api/endpoints/posts/create.js
index 92aeb3d08..eadc886c5 100644
--- a/src/api/endpoints/posts/create.js
+++ b/src/api/endpoints/posts/create.js
@@ -4,7 +4,6 @@
  * Module dependencies
  */
 import validate from '../../validator';
-import hasDuplicates from '../../../common/has-duplicates';
 import parse from '../../../common/text';
 import { Post, isValidText } from '../../models/post';
 import User from '../../models/user';
@@ -32,10 +31,9 @@ module.exports = (params, user, app) =>
 	if (textErr) return rej('invalid text');
 
 	// Get 'media_ids' parameter
-	const [mediaIds, mediaIdsErr] = validate(params.media_ids, 'array', false, [
-		x => !hasDuplicates(x),
+	const [mediaIds, mediaIdsErr] = validate(params.media_ids, 'set', false,
 		x => x.length > 4 ? 'too many media' : true
-	]);
+	);
 	if (mediaIdsErr) return rej('invalid media_ids');
 
 	let files = [];
@@ -135,8 +133,7 @@ module.exports = (params, user, app) =>
 
 	let poll = null;
 	if (_poll !== null) {
-		const [pollChoices, pollChoicesErr] = validate(params.poll, 'array', false, [
-			choices => !hasDuplicates(choices),
+		const [pollChoices, pollChoicesErr] = validate(params.poll, 'set', false, [
 			choices => {
 				const shouldReject = choices.some(choice => {
 					if (typeof choice != 'string') return true;
diff --git a/src/api/validator.ts b/src/api/validator.ts
index 830786a18..3c426054e 100644
--- a/src/api/validator.ts
+++ b/src/api/validator.ts
@@ -1,6 +1,7 @@
 import * as mongo from 'mongodb';
+import hasDuplicates from '../common/has-duplicates';
 
-type Type = 'id' | 'string' | 'number' | 'boolean' | 'array' | 'object';
+type Type = 'id' | 'string' | 'number' | 'boolean' | 'array' | 'set' | 'object';
 
 type Validator<T> = ((x: T) => boolean | string) | ((x: T) => boolean | string)[];
 
@@ -9,6 +10,7 @@ function validate(value: any, type: 'string', isRequired?: boolean, validator?:
 function validate(value: any, type: 'number', isRequired?: boolean, validator?: Validator<number>): [number, string];
 function validate(value: any, type: 'boolean', isRequired?: boolean): [boolean, string];
 function validate(value: any, type: 'array', isRequired?: boolean, validator?: Validator<any[]>): [any[], string];
+function validate(value: any, type: 'set', isRequired?: boolean, validator?: Validator<Set<any>>): [Set<any>, string];
 function validate(value: any, type: 'object', isRequired?: boolean, validator?: Validator<Object>): [Object, string];
 function validate<T>(value: any, type: Type, isRequired?: boolean, validator?: Validator<T>): [T, string] {
 	if (value === undefined || value === null) {
@@ -50,6 +52,14 @@ function validate<T>(value: any, type: Type, isRequired?: boolean, validator?: V
 			}
 			break;
 
+		case 'set':
+			if (!Array.isArray(value)) {
+				return [null, 'must-be-an-array'];
+			} else if (hasDuplicates(value)) {
+				return [null, 'duplicated-contents'];
+			}
+			break;
+
 		case 'object':
 			if (typeof value != 'object') {
 				return [null, 'must-be-an-onject'];