When I try to compile my iOS project I get the following error:
CodeSign error: Certificate identity 'iPhone Developer: System Developer (DF53GDJMSM)' appears more than once in the keychain. The codesign tool requires there only be one.
Most likely you have more than one certificate installed under the same name. Normally this would happen if your certificate expired and you renewed and installed a new one. The one needs to be deleted from the keychain. So launch the Keychain access app and delete the expired or not needed certificate. The close the project, open an compile again. Doing so should resolve this issue.