Q: When I make wudu from a small container, some of the used water usually falls in the original water. Is it problematic for the validity of wudu?
A: The water, used for an obligatory purification, which drips from the limbs is considered pure but not purifying and cannot be used for wudu or ghusl.
A little of the used water droping into that container is fine and will not strip that water of its purifying quality. If however an abundant amount of used water falls back in, it will and therefore cannot be used.
Source: al-Mughni, Ibn Qudamah