Which Link's are the same?

Ocarina of Time + Majora's Mask
Oracle of Seasons + Oracle of ages
Wind Waker + Phantom Hourglass
It is also rumoured that Four Swords Link and Minish Cap Link could be the same.

OoT/MM theory is based on the art style and in-game hints (e.g. Princess Zelda/Song of time flashback)

OoS/OoA theory is based on art style and direct story tie-ins

WW/PH theory is based on the characters and hints towards the previous events.

MC/FS theory is .... yup... you guessed it: how the game looks.

I'm not sure about all the rest, but I'm fairly certain that Majora's Mask Link is the same as Occarina of Time's link. I think I heard that somewhere, and also, he begins Majora's Mask with Epona, and he's not in Hyrule either in this game. Seeing as Zelda more or less gave him his life back at the end of OoT, it makes sense that perhaps he decided to go exploring elsewhere and tumbled into Termina and the Majora's Mask realm.

Its obvious that Orcina of Time and Majora's Mask Links are the same, but the Link in Twilight Princess is ancestor to Wind Waker's.

ocarina of time+ majora's mask.i will think of more later.

4 swords and 4 swords adventure

Spirit tracks and four swords adventure and the minish cap!