I think I have two of the same cat?

Ok this probably sounds weird, I have 4 cats, one of them is a black Persian cat, some times he seems to teleport, he definitely recognises me (if it's him) but I noticed a black Persian cat outside my house ounce, thought it was him, opened the door to call him in, and he was actually sitting in the doorway waiting for me to come home, simular things keep happening more recently. How do i tell if there is actually two of them living in the same house, and tel them apart if they act so simular?