There are 2 considerations for this situation.
1) How similar is the joke?
2) Are you next?
First, you have to determine if it is a joke on the same subject/premise or is it the same TAKE on the same subject/premise. If it is a similar take on the subject, drop yours for that set. If it is just the same subject then evaluate your position.If you are next, open with your joke by using the shared premise as the connector, i.e. “John was talking about Wal-Mart….” If you are not next, just do your joke in its usual position. The audience isn’t like us, they don’t keep a mental register of what’s been done already like comics do. They may notice the topic has been covered already, but it’s a passing thought. Not a judgment on the material, again, like comics do.