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As much as I like Digimon, Pokemon hasn't had to steal/adapt much from them. Pretty much every Digimon game has been different from every other Digimon game (almost none share the same format and battle systems until recently) so if anything, Digimon has become more like Pokemon over time.
Digimon changed from being a digital tamagochi esque pet, to being a "this is a tamagochi and you can only have one, but you can adventure around and try to follow this story", and it has slowly bounced back and forth between ideas (usually related to the Anime series at the time), only to recently settle on a very similar gameplay experience as Pokemon (you have one to three Digimon out at once, it is turn based, you can "catch" more Digimon, there are hold items, etc etc.)
Mega evolution probably did borrow from Digimon, but just as Digimon has adapted to be more similar to the more consistent (and as a result, more popular, Pokemon series), Pokemon has adapted some Digimon concepts and added their own twists (one trio of Pokemon can sort of fuse, Megas require an item to be held, etc.)
Both series have strong points and flaws, so if they can borrow from each other to have fewer flaws, great.
