"For me, that depends.
For example, if you caught a Heatran on your Platinum and you have cloned/traded it around and you decide to rebattle another Heatran, then no. I would not allow anyone to trade different, multiple Heatran with the same OT/ID.
If you caught a Heatran pre RNG days on your Platinum and it has an awful nature and was never traded away, then yes, you may rebattle it to capture a better one.
One legal legendary per game, basically."