Not mine, but a real rivalry between a girlfriend and ex I know a little bit. The guy in question got together with the current gf very soon after the breakup which made the ex suspicious and she's going around asking questions which made the current gf very defensive. The ex has been provoking her, and the gf seems well up for it too, she isn't intimidated.
This would be a pretty evenly matched fight, similar height and weight, both girls go to the gym and work out a lot. The ex girl is more focused on strength whilst the girlfriend works more on stamina an flexibility. Who wins???