I'm currently editing my first novel, "A Day in the Live(s) of Whitaker Riley" and I have a question: In the story, Whitaker (the main character) discovers that there are two other versions of him in the world. My dilemma is whether or not I should try to make up a less-than-cheesy explanation for this bizarre phenomenon or leave it a mystery? An explanation would not be vital to the story, but do you think readers will want to know?

I think you should leave it a mystery. In my experience, readers are more interested in reading a book if there are things about it that they can't understand.

Or you might take a rather Tolkien-like track: Only have a suspicion yourself.

The Palantri, for example, are never fully explained, and Tolkien himself never appears to posses an explanation but instead "suspects" that certain factors may operate in such-and-such manner.

You could just have a suspicions of how the Doppelgangers may operate but not know for sure, or not know the entirety.

As long as you know generally how it works, then you probably don't need it in the story!

Oh, I had forgotten that I posted this! Turns out I decided to make the "doppelgangers" not really other versions of Whitaker. Now they're the result of a... unique family situation, which is a big part of the background to his story.