It happened to occur to me that Yu-Gi-Oh! Gx has not one, not two, not even three but FIVE instances of Duel Monsters themselves straight up duelling.
Don Zaloog (aka one of the Black Scorpions)
Dark Magician Girl
And that’s not even counting the ones encountered in the Dark World/the other dimensions (Sky Scout, Brron for example). And all of them summon themselves (or a proxy of themselves) onto the field(and Yubel duels several times to boot).
I don’t thinks DM, 5D’s, Zexal and Arc V COMBINED have that many.
Gx was very active in the whole ‘Duel Spirits are real’ thing apparently, because the want to get in on the duelling action.