I can understand why people think this but, it's not the case. Sure they both are pink and got lipstick but, their game behavior is different and there's small details that we do not always pay attention to. In the FNAF VR there's differences to them.
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For one Scott has spoken up about Funtime Foxy. His two posts are "Funtime Foxy is still Foxy the pirate" and "Funtime Foxy is an introvert, you know". In the Freddy Files on Funtime Foxy we see "The burnt red has given way to a gleaming pink and white".
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(Examining their in game behavior)

First, let's look at Mangle. Mangle in FNAF 2 he starts in Kids Cove he doesn't go through phases before leaving he just leaves as he pleases. He only comes in office by vent. UCN and VR is same thing he's coming to the office by vent no phases before coming. He plays the static on each cam he goes to in FNAF 2.

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Now for Funtime Foxy he starts in Funtime Cove as he comes out he goes through phases

In UCN Funtime Foxy demands serious attention failing to give him it ends your game. Their jumpscares aren't the same Funtime Foxy are more like Foxy's. I show all three of em.

Only difference is Funtime Foxy is closer but, on right.

It's that same jump out at you. Only one isn't quite opening their jaw to quite bite you.

UCN It's so similer

They both demand lot's of attention and their jumpscare their both twitching.

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Mangle and Funtime Foxy's eyes.

Obviously Mangle

This is Funtime Foxy down here. See how Funtime Foxy's eyes are darker and have a different kind of look verses Mangle's.
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Also slightly going with time line in FNAF 2 we learn on Night 6 they scrap the toys and keep the old ones in hope while closing the pizzeria. In Sister Location we hear from Hand Unit "After the unfortunate closing of Freddy Fazbear's". So obviously the Funtime animontronics are after FNAF 2 if the toys are scrapped then it would make no sense for Funtime Foxy to be Mangle.  I would imagine Sister Location to take the time at either 1990 or 1991 cause the franchise goes three to four years waiting before reopening. In 1993 then the regular gang is back.  Funny thing is if we all were to agree to Funtime Foxy being Mangle then he would be possessed by Susie's dog as well. However, he's not and never will be Mangle.