The game won't let you take a truly duplicate snap. In order to snap one of the following needs to happen:
1) New round;
2) Board looks different;
3) someone (including you) has posted to chat.
[BUT consider possibility that everyone on your team is using this ... You actually could get a bunch of duplicates]
These rules alone would prevent probably 90% of the truly duplicative snap attempts. But there are two options here that may be useful that would get rid of some of the less useful snaps that people wouldn't want to look through.
First, An option to only snap games that have not been snapped yet. Seems simple enough. Before the tool takes a snap it looks to see if there have been any snaps taken. (Most people don't post their snaps publicly, so this would effectively be checking to see if you or your teammates had taken any snaps). This could be called the "Take first snap" option or something.
Second option is the "check to make sure I snapped option." This checks all your games and makes sure that you took a snap at some point after your turn, but won't take a snap if you have. This one is a little more tricky. It would have to check for the time of the end of your turn (kudos if we can do it from the end of your last real action--like skipping fort phase doesn't count--some people snap before clicking end turn when they don't fort). Then it would see if there have been any snaps since then.
Second option part II: you might want to include a check for anyone who is on your team, so that if anyone on your team forgot to snap this will pick it up. (And also run a similar check to make sure that you aren't adding needless snaps.)