MAJOR SPOILERS
I think he was most definitely a twin. Right from the start, Borden had the trick all lined up but said he wasn't ready to perform it yet. He was too poor then to have paid Tesla for a machine (not to mention that Tesla's assisstant denies ever making such a machine for Borden). Plus Borden admits that he sent Angiers on a wild goose chase after Tesla.
More importantly, I think we see that the two Borden's are totally different, whereas Angiers and his clones are so similar that it doesn't make a difference which of them dies. But there's a more risky Borden, who loves Olivia, keeps going after Angiers, ties the dangerous knot, and enRAB up in prison, then there's the sensible Borden who loves Sarah and genuinely doesn't know which knot was tied.
Plus, if Borden had gone to Tesla for a clone and then sent Angiers after him, he wouldn't spend so long agonising over how Angiers was doing the trick. There was a scene where he shouted at his disguised brother "Why can't you figure it out?" and the frustration seemed very real to me.
My question in return is: did Michael Caine know about the twin? He didn't seem surprised at the end, and he had always said it was a simple trick with a double, but did he actually know?