I am having trouble replicating the problem, but from what I am seeing seems to suggest that an operator has somehow caused a process to run twice - my only explanation at this point is that perhaps they have clicked twice on a command button, causing the process to repeat itself (and in doing so, mess up the results and file updates).
When the operator completes the invoice they click on a command button which activates the CLICK Event

eg, What if the operators mouse was programmed to do a double click every time it was clicked once ?

Has anyone else had a similar experience (or am I completely on the wrong track)

Appreciate any feedback - happens infrequently but when it happens I lose an invoice