By default, ForScore will only let you register a shooter once in each division in a given match.  If you want to let shooters register more than once in a division. check the Allow Reentry button in Program Preferences.

When you attempt to register a shooter a second time in a particular division, the system will warn you but permit it:

On the main score entry screen, Reentry listings are flagged with a yellow highlight:

 

And on the shooter entry screens:

 

And results.  In this example, the reentry is included amongst the other shooters.  You can also specify in Program Preferences that reentry shooters be listed at the bottom of each division/classification.