diff --git a/lib/utils/objectUtils.js b/lib/utils/objectUtils.js
index 4de3c2891..664a1d1b4 100644
--- a/lib/utils/objectUtils.js
+++ b/lib/utils/objectUtils.js
@@ -123,13 +123,17 @@ function union(arr1, arr2) {
 }
 
 function unionSmall(arr1, arr2) {
-  const all = arr1.slice();
+  const all = [];
 
-  for (let i = 0, l = arr2.length; i < l; ++i) {
-    const item = arr2[i];
+  for (let i = 0, l = arr1.length; i < l; ++i) {
+    if (all.indexOf(arr1[i]) === -1) {
+      all.push(arr1[i]);
+    }
+  }
 
-    if (all.indexOf(item) === -1) {
-      all.push(item);
+  for (let i = 0, l = arr2.length; i < l; ++i) {
+    if (all.indexOf(arr2[i]) === -1) {
+      all.push(arr2[i]);
     }
   }
 
diff --git a/tests/unit/utils.js b/tests/unit/utils.js
index b0fb68864..426561b7d 100644
--- a/tests/unit/utils.js
+++ b/tests/unit/utils.js
@@ -12,7 +12,7 @@ const {
 const { range } = require('lodash');
 const { compose, mixin } = require('../../lib/utils/mixin');
 const { map } = require('../../lib/utils/promiseUtils');
-const { jsonEquals, uniqBy } = require('../../lib/utils/objectUtils');
+const { jsonEquals, uniqBy, union } = require('../../lib/utils/objectUtils');
 
 describe('utils', () => {
   describe('mixin', () => {
@@ -450,4 +450,10 @@ describe('utils', () => {
       ).to.eql(items);
     });
   });
+
+  describe('union', () => {
+    it('does not keep duplicates from the first array', () => {
+      expect(union([1, 1, 2], [2, 3])).to.eql([1, 2, 3]);
+    });
+  });
 });