To Confuse Someone With/For Someone Else

We use “confuse someone with someone else”. We do not use “confuse someone for someone else”.

I confused Jane with her twin sister Jessica. 

I always confuse John with Peter, They look so alike.

Many people use the preposition “for” in this situation but you should use “for” with “mistake”.

I have mistaken you for your twin sister. 

“Mistake someone for someone else” is slightly more formal than “confuse someone with someone else”.

Sorry, I thought you were someone else

If you are ever in a situation when you “confuse someone with someone else”, you can use “Sorry, I thought you were someone else” to let the person know that you were confused.