Opponents using the same mecha.

I love it when it happens in video games

The villain or rival being a dark version of the hero is a pretty generic thing to do. If the mecha have exactly the same capabilities then the fight is pretty much guarantee to just be the same attacks clashing each other until the hero gets enough resolve to overpower his opponent.
The way this can be made exciting is if the relationship between the two characters has been developed enough for the fight to be emotional. GaoGaiGar vs GaoFighGar in Final was good. Domon vs Master Asia was good. Hikaru vs Nova in Rayearth had an interesting dynamic though honestly I don't really remember the actual fight much.
I was gonna cite Kyosuke vs Beowulf in OGIN as a bad example because they didn't even really know each other, but I guess that doesn't count as the same mecha because of all the Einst shit.

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That depends on the skill of the runner

>>14033884
It depends on how it's done. You can have the fight focus less on the actual action and more on the characters, or if you want to focus on the action try to have them have different fighting styles, or make the winner the one who can use their common mech's capabilities more creatively.
If you just hand them two identical mechs and have them duke it out in a normal way then it's boring. As a wise man once said, perfectly symmetrical violence never solved anything.

>>14033884
as long as its not blow-for-blow shit like everything that sucked about build fighters

Mainly its a matter of do they have something different like pilot style one of the neatest Examples was the final fight of Megas XLR showing two more or less the same Mechs with the gap being skill alone and different fighting styles

I like it most when it's a surprise, like Lordgenome pulling out Lazengann at the end of the first season of TTGL.