Slinking Giant

Wither is a static ability. Damage dealt to a creature by a source with wither isn’t marked on that creature. Rather, it causes that source’s controller to put that many -1/-1 counters on that creature. See rule 120.3.

If an object changes zones before an effect causes it to deal damage, its last known information is used to determine whether it had wither.

The wither rules function no matter what zone an object with wither deals damage from.

Multiple instances of wither on the same object are redundant.