I have an array of testcases that I am attempting to add to a testset in Rally using the rally api's.

I iterate through the array and call this method for every testcase in the array. They are all being added to the same testset.

```js
RallyConnect.prototype.addTestCaseToSet = function (tcObjectID, tcRef, tsObjectID, tsRef) {
    return new Promise((resolve, reject) => {

        // check to make sure it doesn't already exist
        rallyApi.query({
            ref: '/testset/' + tsObjectID + '/testcases',
            fetch: "true",
            query: queryUtils.where('ObjectID', '=', tcObjectID)
        }, function (error, result) {
            if (error) {
                reject(error);
            } else {
                if (result.TotalResultCount > 0) {
                   resolve({ tsRef: tsRef, tcRef: tcRef, action: 'exists' });
                } else {
                        rallyApi.add({
                            ref: tsRef,
                            collection: 'TestCases',
                            data: [{ _ref: refUtils.getRelative(tcRef) }]
                        }, function (error, result) {
                            if (error) {
                                reject(error);
                            } else {
                                resolve({ tsRef: tsRef, tcRef: tcRef, action: 
 'added' });
                            }
                        });
                }
            }
        });
        //});
    });
}
```

I get the following error quite a bit and the process fails

```js
Error: Could not add artifact to collection
    at generateError (C:\src\testing_utility\node_modules\rally\dist\request.js:38:11)
    at Request._callback (C:\src\testing_utility\node_modules\rally\dist\request.js:118:22)
    at Request.self.callback (C:\src\testing_utility\node_modules\rally\node_modules\request\request.js:187:22)
    at emitTwo (events.js:125:13)
    at Request.emit (events.js:213:7)
    at Request.<anonymous> (C:\src\testing_utility\node_modules\rally\node_modules\request\request.js:1044:10)
    at emitOne (events.js:115:13)
    at Request.emit (events.js:210:7)
    at Gunzip.<anonymous> (C:\src\testing_utility\node_modules\rally\node_modules\request\request.js:965:12)
    at emitNone (events.js:110:20)
  errors:
   [ 'Could not add artifact to collection',
     'Concurrency conflict: [Object has been modified since being read for update in this context] - ConcurrencyConflictException : Modified since read on update
: Object Class : com.f4tech.slm.domain.TestSet : ObjectID : 203533554680' ] }
```

Has anyone else run into this issues and know what I can do to ensure I don't get it.