If a creature has two instances of lifelink does it gain double life?

Lifelink is a static ability that causes damage dealt by the creature to also cause its controller to gain that much life. According to CR 702.15b, the ability simply states that damage dealt by the source causes life gain — it does not multiply or compound.

CR 112.3 confirms that having two instances of a static ability that does not use a cumulative effect does not change the outcome. Lifelink falls into this category: two instances do not mean the damage triggers life gain twice, because the effect is simply 'on' or 'off.' A second lifelink instance is redundant, exactly like having two instances of flying (CR 112.3m).

For example, if your 3/3 creature has lifelink twice and deals 3 damage, you gain 3 life — not 6. The second instance of lifelink is completely irrelevant to the outcome.

This is different from triggered abilities (like 'whenever this creature deals damage, you gain that much life'), where two separate triggered abilities would each independently trigger, causing you to gain life twice. Lifelink, being a static ability, does not work that way.