It may be matching the email address or another "field" and you get to examine all the fields to see where it is complaining.
Not a bug, but an annoyance.
I sent someone an email. I clicked on their address to add to the address book - error message - the contact is already in your address book. No it's not. It's not there. I went to Sent items and again tried to add them. Same message. Where is it?
I tried closing program and reopening. no difference.