Is it possible to recover overwritten reports?

Hello,

I'm carrying out a study (still the same one!) where the data in the report file is crucial to analysis, as it shows the scenario that was presented to the participant. I've noticed that if i assign the same number (accidentally) to participants, the first report with that number is overwritten- or at least disappears. Is there any way to recover it, or any way to ensure that i don't use the same number twice other than manually keeping a record of it?
Thanks!
Lindsey