this is normal because of anti-aliasing (semi transparent pixels) around the edges. please explain what are you trying to accomplish so we may give you workflow advice. if needed, show some screenshots.
Have you tried pre-composing the layer & duplicating the composition? It seems reasonably logical that a duplicated layer would be identical but After Effects does everything halfway & backwards so....I wish I could give more advice but I don't have a ton of experience with keying.
Ha. You see. It's normal. LOL
Are you still facing this problem? If not, let us know how you solved it. If so, please let us know so we can assist you further.