Having more than one Plague Drone on the battlefield doesn't have any noticeable effect on life gain. Once the effect of one Plague Drone applies, there is no life gain for the others to apply to.

If more than one replacement effect tries to apply to a life gain event, the player who would gain life chooses the order in which they apply. For example, if a player who controls Alhammarret's Archive would gain 3 life while Plague Drone is on the battlefield, that player may choose to have the 3 life become doubled to 6 life and then lose 6 life. The player may also choose to apply Plague Drone first, turning "gain 3 life" into "lose 3 life." Alhammarret's Archive would then not apply.