On the issue of now and after 30 days

Hello Rabbi!
In the Kiddushin 52-61, the discussion revolves around a reality in which several people sanctify one woman. The Rashba offers the following understanding of the opinion of Rabbi Yochanan:
"But it is clear from Rabbi Yochanan that if God wills, then you are consecrated to me from now on and after death, then it is consecrated, except that other sanctities will be taken from it forever, a concession to anyone who sanctifies it until after death… and so every thirty days is bought, even though there is a remnant in it, and if another comes and sanctifies it, it is consecrated for two times, even for the second time, even if God has sanctities, all of which are purchased."
My simple understanding was that once there is a Kiddushin, then as a direct result of these Kiddushin, the woman is not eligible for Kiddushin for another person. Is this the correct understanding, and according to this, how can we reconcile the Rashba's words? (In general, from the first in the issue, it follows that there is a halachah for the Kiddushin of the first, but there is 'room left' for other Kiddushins within the day.)