Probably everybody has seen individual episodes of certain TV series at least twice. For example, most people have probably seen a few episodes of M*A*S*H more than once. However, there are certain TV series that I liked so much that I had to see the entire series at least a second time. Since I am a such huge fan of science fiction, most of the series I watched were science fiction or fantasy:
Battlestar Galactica (2003-2004): Twice.
Stargate Atlantis: Twice.
Stargate Universe: Twice.
Star Trek Enterprise: Twice.
The Event: Twice.
Flash Forward: Twice.
The 4400: Twice.
Stargate SG-1: At least twice. Parts of it I have seen three times.
Doctor Who (2004): At least twice.
House M.D.: Working on the third time.
Star Wars The Clone Wars: Working on the third time.
The Walking Dead: Working on the third time.
Buffy The Vampire Slayer: At least three times.
(Although the series starts out uneven, the later seasons are really brilliant. If there is a single episode of any TV series worth watching multiple times, it is "Once More with Feeling" from the 6th season of Buffy. You could probably watch this without seeing the rest of the show, although it would be better to have seen the series first.)
Lost: Working on the 4th time.
I would like to watch Star Wars Rebels for a second time. It has improved with each new season.
You might think from the list above that I do nothing but watch television all day, but if you watch only 1 or 2 episodes per day, then you can quickly run out of things to watch.
For the new shows on TV, the only ones I am watching right now are Colony, The Expanse, Star Wars Rebels, and The Walking Dead.
I especially like rewatching shows while I do my 2 hours of physical therapy everyday.