Multiple instances of lifelink aren't cumulative. Activating Arctic Aven's ability more than once during a single turn won't cause you to gain more life.
Flying
This creature gets +1/+1 as long as you control a Plains.
: This creature gains lifelink until end of turn. (Damage dealt by this creature also causes you to gain that much life.)
: This creature gains lifelink until end of turn. (Damage dealt by this creature also causes you to gain that much life.)
