Carbon Copy episodes.

[The Abominable Dr. Lenny]

So I wouldn't consider a lot of these copies, rather just episodes with similar plots.

 

[The Abominable Dr. Lenny]

I was gonna say Lisa's Rival and The Kid Is All Right but it's been taken already.

But those two explore the same idea, but take it in different directions. How many possible ideas can you have in a show centered around the same roster of characters?

 

[Trab Pu Kcip]

At least 3. LR, LOTD and TKIAR.

 

[The Abominable Dr. Lenny]

Yeah, but all those episodes explore Lisa having an adversary of some sort, but do it in different ways.

 

[John Smith 1882]

Saying it like that makes it sound worse than it is.

Well, you can't deny both episodes were pretty bad.

 

[The Abominable Dr. Lenny]

Well, you can't deny both episodes were pretty bad.

Oh, yeah. They both sucked hard.

 

[714MatchesFound]

Lisa the Vegetarian: Lisa becomes disillusioned from eating meat and seeks an alternative diet, vegetarianism. After fighting with her family, she runs off and receives advice from a celebrity. Lisa stays this way for the rest of the series, but learns to respect other people’s opinions, and vice-versa.

She of Little Faith: Lisa becomes disillusioned from the church selling out and seeks an alternative religion, Buddhism. After fighting with her family, she runs off and receives advice from a celebrity. Lisa stays this way for the rest of the series, but learns to respect other people’s opinions, and vice-versa.

 

[Trab Pu Kcip]

This one I consider similar but not a carbon copy.

Bart Star: Parents wants them to play in Pee Wee Football. Team goes on winning run. Homer ruins it and they lose. The people who were in a argument forgive and they win the championship.
There Will Be Buds: Exactly as above, but with lacrosse.

 

[ManuelMaiden95]

Lisa the Vegetarian: Lisa becomes disillusioned from eating meat and seeks an alternative diet, vegetarianism. After fighting with her family, she runs off and receives advice from a celebrity. Lisa stays this way for the rest of the series, but learns to respect other people’s opinions, and vice-versa.

She of Little Faith: Lisa becomes disillusioned from the church selling out and seeks an alternative religion, Buddhism. After fighting with her family, she runs off and receives advice from a celebrity. Lisa stays this way for the rest of the series, but learns to respect other people’s opinions, and vice-versa.

I've always said She of Little Faith is a poor rehash of Lisa the Vegetarian.
The only difference is that Lisa the Vegetarian was actually funny, the social commentary was spot on, the episode was well structured and the resolution with Homer and Lisa reconciling was believable and touching.
She of Little Faith hasn't any of that, Marge is way over the top and comes out as an intolerant jerk and the structure of the episode is a complete mess. We learn it's a Christmas episode in the last act and it comes out of nowhere.

 

[John Smith 1882]

Pulpit Friction: A younger, hipper preacher voiced by a guest star comes to town and the townspeople like him better than Lovejoy, Lovejoy is forced out of the church, but at the end the townspeople decide they want Lovejoy back and the new guy is forced out and is never heard from again.

Warrin' Priests: A younger, hipper preacher voiced by a guest star comes to town and the townspeople like him better than Lovejoy, Lovejoy is forced out of the church, but at the end the townspeople decide they want Lovejoy back and the new guy is forced out and is never heard from again.

 

[Trab Pu Kcip]

If we're allowed to count segments of Halloween episodes then:

Send In The Clones: Homer uses a cloning mechanism to clone himself. He hangs out with the clones. All (including Homer) but one of the clones dies. The remaining one makes out with Marge at the end.
Into The Homerverse: Homer uses a cloning mechanism to clone himself. He hangs out with the clones. All (including Homer) but one of the clones (who turns into another clone) dies. The remaining one makes out with Marge at the end.

I swear they're the same premise...

 

[DigiJem1]

The 7 Beer Itch: Homer finds an attractive woman and tries to not cheat on Marge.
The Last Temptation of Homer: Homer finds an attractive coworker and tries not to cheat on Marge.

 

[John Smith 1882]

That's a carbon copy of many, many more episodes than just that.

 

[John Smith 1882]

Dad Behavior and Better Off Ned were both carbon copies of Brother From The Same Planet.

 

[SuperNintendo_Chalmers]

I find Lisa with an 'S' and Gal of Constant Sorrow very similar but not a carbon copy. Old woman befriends Lisa and tries to get her into the showbiz/music business and ends up betrying her.