All four are completely different (well, except for being based on the same movie for each pair):
DHS Little Mermaid and AK Nemo are both shows. Little Mermaid DHS features live actors and animatronics, and mostly follows the storyline and songs of the movie. Nemo is entirely done with Broadway-caliber puppets, follows the storyline of the movie, adds new songs (written by the same people who did Avenue Q on Broadway), and is really just a must-see show,
MK Little Mermaid is an animatronic dark ride, following the storyline of the movie; it's a fairly short ride.
Epcot Nemo is an entire pavilion containing:
1. A dark ride through a "sequel" where Nemo gets lost again and goes through the same sorts of scenes as the movie, ending with scenes projecting the characters into an aquarium with real marine life.
2. The rest of that aquarium, with lots of different smaller exhibits to look at.
3. Turtle Talk with Crush, a show with "live animation" like the Monsters Inc. Laugh Floor in MK, where kids in the audience get to ask Crush questions.
If the line's too long at MK, you might consider skipping that ride, but I wouldn't skip any of the other three, and you shouldn't be worried about the overlap between the four rides/shows/pavilions.