Maybe I'm out of the loop but I was always of the opinion that "ikr" means "I feel the same way". I was taught as a kid not to say "I know" because "it's rude"... I'm not sure that's true but it's given me the chance to find lots of alternates.
The Urban Dictionary page for "I know, right" seems to agree with this. The top definition is
It's complete agreement in what is being said.
So, alternates would be:
I completely agree.
I've always felt that way, too.
I feel the same way.
We're on the same page.
I think that all of these imply to some degree that you already know what they're saying and if you want to be clear, you can combine them.
I completely agree... actually, I've always felt that way, too.
Granted, these are somewhat more formal than "IKR" or "I know, right". But if you are in a sufficiently informal situation you can probably simply say "IKR" or "I know, right".