"YOU'LL BECOME APOCALYPSE'S GREATEST HORSEMAN!: Give Genesis a free action and choose another friendly character within range and line of fire. Until your next turn, that character has the Horsemen of Apocalypse keyword and modifies all of its combat values by +1, unless already modified by this effect."

If the character chosen for this power already has the HoA keyword, would it still get the combat value bonus?

I don't see why not.

It's like having the keyword twice without any obvious bonuses...besides this one.

Agreed. Just like a Hammer can give you Charge and +2 AV... if you can already use Charge, you still get the +2 AV regardless.