Deckard is a Replicant!!!
1. The unicorn dream being known by Gaff. Gaff knew deckard's dream, just like deckard knew Rachael's memories. He had access to his file.
2. Gaff saying "You've done a MAN's job, sir." Then immediately saying "It's too bad she won't live, but than again who does."
Now that's 2 questionable comments in the space of 2 min. I would only make that statement to someone if I meant to imply that they won't live as well. I would find it very unlikely that at such a moment as that, Gaff would be engaging in small talk.
3. Rachael asks Deckard, "You know that Voight-Kampff test of yours? Did you ever take that test yourself?" deckard then falls asleep and never answers that crucial question... next scene his eyes glow as he tells Rachael he wouldn't come after her.
4. When Roy catches Deckard in mid air on the building ledge he shouts," Kinship!" Now why on earth would a replicant that defiantly knows it's a replicant shout that out to a human who is not it's kin???
5. Deckard and Rachael both play the SAME tune on the piano !!!! An implanted memory by the engineers!!
All the clues are there... Deckard is a REPLICANT!