Multiple instances of lifelink on the same creature or planeswalker are redundant.
2019-05-03
If Sorin leaves the battlefield after you've activated his first loyalty ability during your turn but before it resolves, you'll still gain 1 life as that ability deals damage.
2019-05-03
If a card in a graveyard has in its mana cost, X is considered to be 0.
During your turn, creatures and planeswalkers you control have lifelink.
+2: Sorin deals 1 damage to target player or planeswalker.
−X: Return target creature card with mana value X from your graveyard to the battlefield. That creature is a Vampire in addition to its other types.
in its mana cost, X is considered to be 0.