In a fandom that's so fond of shipping, it's inevitable: somepony is going to be shipped with themselves.
This group is for stories that do so.
Rule 63 -- Ponies with genderbent versions of themselves--for instance, Twilight Sparkle/Dusk Shine or Rarity/Elusive. These pairings are M/F by definition.
Cloneshipping -- "My own clone! Now neither of us will be virgins!" The classic joke taken to its logical conclusion. A pony makes a clone of themselves... and gets distracted somewhere along the way. These pairings are M/M or F/F.
Time/Dimensional Travel -- Imagine if Twilight had traveled back in time to warn herself not to worry about the future... and then noticed for the first time how attractive she was. Again, these will be M/M or F/F.
Psychosis -- Dissociative Identity Disorder gone terribly, terribly right. Examples are Pinkie Pie/Pinkamena or Nightmare Moon/Luna. Can be M/F, M/M, or F/F.
Changelings -- Changelings are shapeshifters. This is a selfcest group. See where I'm going with this?