Mistaken Identity
noun, verbal noun
A person who is different from the person you thought it was is a mistaken identity. Whoops, sorry, I thought you were someone else. 人違い!
The readings are 人 and 違い put together.
はい、私達も結合双生児ではありますが、申し訳ありませんがあなたは人違いをしていると思います。
Yes, we are also conjoined twins, but I’m afraid you are mistaking us for a different set of twins.