This card's effect is a Continuous Effect that cannot be chained to.

The 400 points of damage is inflicted after the effect of the Normal Spell Card is resolved. So if the opponent had 300 Life Points, and you activated "Rain of Mercy" while you had "Rapid-Fire Magician" face-up on the field, the opponent would gain 1000 Life Points, and then take the 400 points of damage, leaving them at 900 Life Points.

Doppelganger: If you take damage from multiple monsters' Continuous Effects at the same time, like "Rapid-Fire Magician", "Doppelganger's" effect only activates once, and inflicts damage equal to the damage from only 1 of those monster's Continuous Effects.