Please also see Paul McCartney's voice vs. Faul's voice.
To the left is a vocal comparison of "Eleanor Rigby. Below it is yet another voiceprint comparison of Paul and Faul.
Paul McCartney singing his classic, "Yesterday":
Faul singing "Eleanor Rigby:
Rotten Apple 51:
Methods such as these made it more difficult for people to recognize it was not Paul singing. When you can focus on the voice, the difference is striking.
Unfortunately, it seems that technology can mimic a person's voice making it more difficult to identify a double using voiceprints.
Please also see Paul McCartney's voice vs. Faul's voice and Tina's book for more information: The Splitting Image: Exposing the Secret World of Doubles, Decoys, and Impostor-Replacements.