Hi Thai,
I have also noticed this. The causr is that persons are able to submit the timesheet twice. The first approval will post the timesheet. The second will raise the error. You can create an incident at Microsoft support for this. My customer did not do it as they have other priorities.
There are fixes for similar workflow problems on LCS, but not yet for the timesheets.
When you click the submit button from the AX client and before click OK also a second time, it is quite easy to submit twice.