What is wrong with my brother?

This is a little hard to explain, but for a while my brother thinks he's done things that I've done, such as telling a joke one time and claiming it was him who told it when I was the one who actually said it or just about anything. It's like he thinks he was me in a moment in my life, but thinks it was himself instead of me, meanwhile he was someplace else. Actually it exactly is that way. He sometimes thinks that what I do, is actually what he did, not in spite of anything.