If a creature you control already has lifelink, Titanic Ultimatum will give it a second instance of lifelink; however, multiple instances of lifelink on the same creature are redundant. You still only gain life equal to the amount of damage dealt, regardless of how many instances of lifelink the creature has.

The set of creatures affected is fixed as Titanic Ultimatum resolves.

Multiple instances of first strike and trample are redundant.