Did you know that has a show gets older. It runs out of ideas and has episodes that are ripoffs episodes from early in the serious.

I just watched one tonight on Nick at Nite. The Cosby Show ep "The Birthday Party" is very much a ripoff from the Cosby Show ep "Slumber Party". Basiclly Cliff entertaining little kids at a party and having kids show off their talents. Infact both episodes had twin girls playing patti cake.

Also Diff'rent Strokes did a same a couple of times. The DS ep "Tonsils" where Sam gets his tonsils taking out his a rippoff of the DS ep "Arnold's Girlfriend" where Arnold gets his apendex taking out. Also the DS ep "Sam's Bedwedding" is a ripoff of the DS ep "No Time for Arnold" in which both ep deals with bedwedding.

I remember a couple times where Lucy and Ethel would dress up in disguises to try to trick Ricky and Fred.

The episode where Lucy was in the short Italian wig and Ethel couldnt get the complete costume (part Eskimo, part Geisha, and part Indian (Native American) was great.

The other episode where they were hillbillies or hicks was not good. It must have been the first season.

Bewitched did that several times. Most of them were instances in which the plot from an older episode with Dick York was recycled and used in a Dick Sargent episode. For example, the first episode ever filmed with Dick Sargent (#183 Samantha's Better Halves) used the exact same plot as a Dick York episode (#67 Divided He Falls), in which Endora splits Darrin in two to be with Sam and help Larry on a business deal at the same time.
Side note: Although this was the first he filmed, it was aired out of order in midseason.

The Three's Company episode "No Children, No Pets" was ripped off in the last season of the show with an episode called "Look What I Found." Both episodes had the roomates finding a tiny animal (in NCNP it was a puppy, in LWIF it was a kitten) and since they don't have the heart to keep it, they try to hide it from the landlords.

A classic sign the the show is getting old and ready to be put out to pasture is when they start repeating themselves. I remember an episode of MASH where Hawkeye and Trapper are circulating a pair of long johns in the early days, then on one of the last ones Hawkeye and BJ are circulating a bathtub around to beat the heat. It happens to all shows no matter how good they are. I suppose it is a nice problem to have because it means you stayed on long enough to repeat yourself.

I know that technically, it's considered a different series, but "Saved by the Bell: The New Class" did a lot of plots done by the original class.