How do you say ive seen this before in french?

There are a few ways to say "I've seen this before" in French, depending on the context and level of formality:

Formal:

* J'ai déjà vu cela. (Most literal translation)

* Je connais déjà cela. (I already know this)

Informal:

* Je l'ai déjà vu. (I've already seen it)

* C'est déjà vu. (This is déjà vu) - implies a sense of familiarity or repetition.

* J'ai déjà vu ça. (I've already seen that)

More specific:

* J'ai déjà vu ce film. (I've already seen this movie)

* J'ai déjà entendu cette histoire. (I've already heard this story)

The best choice depends on the specific situation and your relationship with the person you are speaking to.