Handle duplicates of the allowed values

lib/assertions.js

@@ -786,7 +786,15 @@ module.exports = (expect) => {
             );
           });
           if (expect.flags.only) {
-            expect(subject.length, 'to be', args.length);
+            expect(
+              subject &&
+                Array.prototype.every.call(subject, (item) =>
+                  Array.prototype.some.call(args, (arg) =>
+                    expect.equal(item, arg)
+                  )
+                ),
+                'to be truthy'
+            );
           }
         },
         (e) => {

test/assertions/to-contain.spec.js

@@ -179,6 +179,17 @@ describe('to contain assertion', () => {
       }, 'not to throw');
     });
 
+    it('should not throw when all items are included in the subject and some multiple times', () => {
+      expect(function () {
+        expect(
+          [{ bar: 456 }, { foo: 123 }, { bar: 456 }],
+          'to only contain',
+          { foo: 123 },
+          { bar: 456 }
+        );
+      }, 'not to throw');
+    });
+
     it('should throw when all items are not included in the subject', () => {
       expect(
         function () {