re-using water from netilat yadayim

Question

k'vod harav. Near to his sukkah, my `havruta uses a big vessel full of water and guests use a keli, taking the water from the vessel, doing netilat yadayim (which pours in the vessel)... hence, a part of the water is re-used water from the netila, for the next netilat yadayim. Is this kasher? Is it a matter of proportion of the volume of the keli to the volume of water originally in the vessel? Please comment.