You almost certainly aren't going to see the show.
In theory it should be simple to identify who you are in conflict with if the ticket is not general admission. They would just have to send an usher to the seat. However, it is difficult to determine who defrauded who without a whole lot of investigation.
You would have to establish the entire chain of custody for both of the tickets. Three possibilities with different culprits are: the original purchaser double sold the tickets, a third-party got the barcode and sold a fraudulent ticket to whoever is in your seat, or the person in your seat made the fraudulent ticket themselves.
In theory it should be simple to identify who you are in conflict with if the ticket is not general admission. They would just have to send an usher to the seat. However, it is difficult to determine who defrauded who without a whole lot of investigation.
You would have to establish the entire chain of custody for both of the tickets. Three possibilities with different culprits are: the original purchaser double sold the tickets, a third-party got the barcode and sold a fraudulent ticket to whoever is in your seat, or the person in your seat made the fraudulent ticket themselves.