[Original bug] Gunner can throw 2 grenades when ducking

[BjossiAlfreds]

On hard and nightmare the Gunner has a 50% chance of throwing a grenade when ducking down. This works correctly in the other games but in GroundZero, rogue's new ducking code introduced a bug. Because gunner_duck_down is called twice, first from M_MonsterDoddge and again from the ducking frameset, GunnerGrenade gets called as many as 2 times with only 1 frame delay (1st one spawns during ClientThink and the other during next G_RunFrame when the Gunner thinks).

In the other games the chance of a grenade is 50% but in GroundZero it is 75% - 50% 1 grenade and 25% 2 grenades.