This is a fantastic question- and one that had me pondering for a bit before I found my answer.
If we’re talking about just general full moon energy, then I’d agree with what @phoenix_dawn said- if you feel something (moon water, in this case) is fully charged and you plug it back into the charger (so to say lol), well, it’s not going to get any more charged that it already is .
However, many witches view each of the 13 full moons of the year as having unique energies. The Full Sturgeon Moon yesterday, for example, carried the energies of decrease and banishment- traits not usually associated with the Full Moon that were unique to the August moon.
For those that seek the unique energies of each moon in their moon water, then I think absorbing the water of multiple full moons would perhaps cancel out contradicting traits- the full moon of new beginnings would cancel with a moon of banishment
However- if someone were to charge their water under all 13 full moons of the year, then I feel like that water would take on an entirely new property, unique from the traits of the individual moons themselves. Moon Water of all 13 Full Moons could represent a complete cycle of the year- some incredibly powerful moon and time energy
13 Moons Moon Water could thus be used for: moon magick, time magick, spellwork dealing with the year, rituals related to the seasons, enhancing spellwork
This is all hypothetical, as I have never charged moon water for 13 full moons in a row. If anyone has (or does)- then please share your experience, I would love to hear about it!
Thanks again to welcome for such a great and thought-provoking question!