Working on a new project that is just getting its parameters setup and got into this issue when trying to post a journal: "Posting - Payment journal Journal: USMF-000010 Voucher: USMF-000100 Voucher USMF-000100 is already used as of date 08/01/2016. Posting has been cancelled.".
The answer to this is quite simple and it lies in the fact that the company has some setup issues: it is trying to use the same voucher value that it gets from a number sequence using the same format as another one, and just got here into a collision.
You should review the setup you have for the journal's voucher number sequence and make the necessary changes so the number sequences formats are different.