In situation comedies, there tends to be certain patterns that emerge as you watch characters traverse the world of relationships. One which I've noticed popping up more often than I had thought is the relationship doppelganger. To explain that simply, I mean instances where one character begins dating someone who is clearly a younger or slightly altered version of another character on the show.
I have broken these kinds of relationships into 4 different kinds.
The Oedipal: Examples: Frasier, Two and a Half Men
The lead character has just met the perfect woman, someone who makes them feel something they haven't felt in a long time. And then the other characters on the show meet the new girl and there is a stunning physical and at times, personality similarity to the lead character's mother. Everyone else can see it and is unnerved by it, but the character in question is in denial. It finally takes seeing their new lover in a situation which makes it clear that they are very much the embodiment of their mother for them to finally understand what they have done.
In the case of Frasier, it took seeing some home movies of his mother, played by the same actress as his character's girlfriend, Rita Wilson, for him to finally see that he was dating his mother. And on Two and a Half Men, Charlie had to see his current girlfriend with her two sons to finally see the parallels between her and his own mother. The fact that he stays with her after noticing it pushed the joke way too far though.
The Ex/Desired Lover: Examples: Friends, Frasier
A character either breaks up with another character on the show or they secretly long to be with them, and they then start dating someone who is very similar physically and in terms of personality to that person.
With Friends, there was a single episode which followed Rachel's admission that she wanted to be with Ross, and the ill fated list that angered her so. So she ends up dating Russ, who is also played by David Schwimmer. Ross hated Russ, even though all the things he hated were things that he himself did. Of course, everyone but Ross and Rachel could see from the outset that they were very much the same person, which said everything about Rachel's feelings for Ross.
And on Frasier, just before Niles was going to ask Daphne out, she met Rodney at a singles bar, a man who looked and acted very much like him. Unlike Ross however, Niles almost immediately realizes the similarity. In the end, Rodney ends up falling in love with the woman that Niles had started dating and in the final segment, Niles ends up comforting Daphne about the breakup.
And these last two examples are both Seinfeld spins on this kind of thing.
Dating Your Best Friend: George dated a girl who physically looked a little like Jerry. It wasn't until she got gum stuck in her hair and had to get a short haircut that he finally saw it.
Dating Yourself: Jerry got engaged to a girl played by Janeane Garofalo who was basically him as a woman. He decided that he couldn't date himself because he hates himself.
Now, there are likely other variations on this theme that I've missed, but I think you get the basic idea of this particular category.