123entered - Do not allow a subsequent end-of-work press during the backflush period

If the back-entry period is on, you can still set the end of the working day for the lecture the next day, for example. In my opinion, this is not good. Back entry is good if you have to add expenses or, for example, subcontractor entries. But if, for example, I go to yesterday today and press end of work, I press end of work for the same time yesterday. In my opinion, this should not happen and the function should be adapted.