groupMembers: add unique constraint

fixes #696

src/test/groups-test.js

@@ -224,6 +224,13 @@ describe('Group membership', function () {
         });
     });
 
+    it('cannot add same member again', function (done) {
+        groups.addMember(group0Object.id, USER_0.id, function (error) {
+            expect(error.reason).to.be(BoxError.ALREADY_EXISTS);
+            done();
+        });
+    });
+
     it('can add member without username', function (done) {
         groups.addMember(group0Object.id, USER_1.id, function (error) {
             expect(error).to.be(null);
@@ -270,13 +277,20 @@ describe('Group membership', function () {
         });
     });
 
-    it('can set groups', function (done) {
+    it('can set members', function (done) {
         groups.setMembers(group0Object.id, [ USER_0.id ], function (error) {
             expect(error).to.be(null);
             done();
         });
     });
 
+    it('cannot set duplicate members', function (done) {
+        groups.setMembers(group0Object.id, [ USER_0.id, USER_0.id ], function (error) {
+            expect(error.reason).to.be(BoxError.CONFLICT);
+            done();
+        });
+    });
+
     it('can remove member', function (done) {
         groups.removeMember(group0Object.id, USER_0.id, function (error) {
             expect(error).to.be(null);
@@ -352,6 +366,13 @@ describe('Set user groups', function () {
         });
     });
 
+    it('cannot set user to same group twice', function (done) {
+        groups.setMembership(USER_0.id, [ group0Object.id, group0Object.id ], function (error) {
+            expect(error.reason).to.be(BoxError.CONFLICT);
+            done();
+        });
+    });
+
     it('can set user to multiple groups', function (done) {
         groups.setMembership(USER_0.id, [ group0Object.id, group1Object.id ], function (error) {
             expect(error).to.be(null);