Parallel file upload fails

My bad, the 403 response was not for these requests. However some files are still missing. It can be reproduced with this:

#!/bin/bash
for i in `seq 10 19`;
do
curl -H "Authorization: Bearer <my token>" https://api-content.dropbox.com/1/files_put/auto/ -T file$i &
done;

Now I can see that the response for the missing files is:

{"error": "Failed to grab locks for 532934897: lock held by connection 4695316.Please re-issue the request."}

Is there a queue I can use on your side for file upload? Else I guess I have to check the response and retry if necessary. Thanks!