He was head-over-heels in love with Claudia, his girlfriend of seven months, convinced that things between them were humming along seamlessly.

So it came as a complete shock when she suddenly dumped him and, less than a week later, took up with another man.

Derek was devastated. He didn't just adore Claudia, he worshiped everything about her: her long, flowing blonde locks; her high cheekbones; her cynical, cutting sense of humor; her fanaticism for British comedy troupe Monty Python -- which he shared.

From Derek's perspective, their intimacy was unlike anything he had previously experienced: they generated plenty of sparks in the bedroom and perfectly fulfilled each other's needs (or so he thought).

Even their friends approved: Claudia's thought he was a prize catch. Derek's approved of the positive influence she exerted in his life.

But the manner in which it came crashing down and the unlikely prospect of a reunion -- now that Claudia had a new Romeo in her life -- prompted an earnest resolve in Derek to get back on the horse as quickly as possible.

He was convinced that he could recapture the magic with someone else: It was time to find another Claudia.

The phenomenon of dating the doppelganger -- a person who closely resembles your ex -- is more common than you might expect, says Dr. Yvonne K. Fulbright, a Scandinavian sexologist, relationship expert, advice columnist, media personality and author of Sex with Your Ex and 69 Other Things You Should Never Do Again...Plus a Few that You Should.

"It might present itself in how somebody looks in appearance, or go as far as their personality," Fulbright says. "Of course, the more you have both of those coming at you from the same person, the more of a doppelganger situation that you have."

Relationship coach and clinical sexologist Sandra Reishus -- author of the books 'OH, NO! I've Become My Mother' and OH, NO! He's Just Like My FATHER -- says it's only natural to cling to what you perceive as a happy situation.

"If something is good, then people want it again, and they want more of it," notes Reishus.

Doppelganger dating is often, though not always, a response to being on the rebound.

Fulbright believes that the person who suffers a "heart-breaking breakup, especially the one who's been dumped" is more vulnerable to winding up in this type of situation.

And we are often blissfully unaware of our doppelganger dating behavior.

"I think a lot of times when a person is feeding the fuel and they are rebounding, clearly their blinders are on," says Dr. Fulbright. "You're just looking for comfort, solace, just some way to cope and move forward.

"In a way, trying to talk to somebody as far as awareness about this goes is like trying to tell the alcoholic not to have another drink when they haven't been to an AA meeting yet.

"Most people that are in love and have been broken up with are going through drug withdrawal. That's what brain research shows: pretty much the same areas of the brain are being affected where the withdrawal process is almost like coming off of a drug addiction.

"Part of that is moving into that state of fantasy or denial. For people on the rebound, going for that doppelganger relationship is a pretty standard way of coping."

Identifying this pattern -- especially if you're the new target -- shouldn't be that difficult. If you're the perpetrator and can't see it, usually your close circle of friends will drop a hint or two.

"Usually your good friends will say things like, 'Oh, I can't believe that he look so much like so and so,' or 'The way he does this really reminds me of so and so, ' says Fulbright.