CodeSign error: Certificate identity \'iPhone Developer: XXXX (12345678)\' appears more than once in the keychain. The codesign tool requires there only be one.
So I
I just had the same problem. It has been fixed. It came after I submitted an app and installed a few certificates to sign the app.
Quick Fix: Open up KEYCHAIN ACCESS > click on MY CERTIFICATES> in there you will probably see iPhone Developer: . You will probably see it TWICE! Select the one with the earliest Expiration date, right click and select DELETE.
Restart Xcode if you haven't. Works now. :)
Happy Coding.