But don't it feel wrong to fire multiple defensive-fire actions in one turn?
Multiple assaults on the same unit during the Assault Step should be extremely rare.
If a unit fails to counterattack, they lose the assault and have to Break Off, moving more than 6" from the assaulting teams or they are destroyed. This move can and should move them too far away from another unit that has yet to assault, considering most units can only Charge into Contact within 4" (with the notable exception of some mid war Soviet infantry) and there's a list of other conditions that the would-be assaulting unit will have had to meet (pg
.72). But, in the event that another unit can launch an assault against a unit that broke off, then yes, that unit (and all units within 8") can fire in defensive fire if they are assaulted.