Giving a creature the lifelink ability more than once will not do anything more than if it had the ability once. The ability is not cumulative and creatures that have the ability more than once will not cause their controller to gain more life than the damage dealt.

For example, Moment of Heroism gives a targeted creature +2/+2 and the lifelink ability. If a Markov Patrician is targeted by Moment of Heroism it will become a 5/3, and it will technically have the lifelink ability twice. However when the Patrician deals damage, the controller of the Patrician will only gain five life.