Is it possible that Arthur Fleck is not the real Joker? [duplicate]

(...) no Batman movie/comic book/etc gives the real name of the Joker(...)

Actually the 1989 Batman film tells a origin story of the Joker, whose name would be Jack Napier. From what I've heard this is not grounded on the comics.

Alternatively, there is the Three Jokers Concept and the ideia of Joker precursors used in Fox's TV series Gotham. In both cases, there is the idea that the Joker is not one person, and for different reasons.

So...

Is it possible that Arthur Fleck is not the real Joker?

Yes it is. Particularly:

(e.g. Heath Ledger's Joker)?

Theories for Heath's Ledger's Joker normally revolve around him having a military background. That Joker is also evidently someone with high intellect, who wants to convey a philosophical message, unlike Fleck, who claims he is "not political", and is mostly portrayed as someone who urges to be heard.

Then again, why would Ledger's joker be "the real" Joker and Phoenix not? They are both iterations on a very mutable character. Better than that, I'd say the "Joker" is a honorific character rather than a single character, as explained at 1min08s of this video. Also within this video's line of thought, the producers are exploiting multiplicity, which from the quote in the video "popular media is moving away from a focus on continuity and instead gravitating towards multiplicity".

It's not only possible. It's expected.
Or to be even more "cannon" he's one of the Jokers. There are multiple explanations on "where did the Joker come from and who he is" and each one is/could be real explanation. Which the readers/viewers cannot agree because some of the versions are told by Joker himself. And it don't have anything to do with multiverse but the fact that people lie.

There is a story in one of the Black & White Batmans comics where is shown that one of the Arkhams workers comment on an origin story Joker told Harleen Quinzel as totally fake because it was told by Joker.