I've been tasked with managing the assignment of keys to our facility. Yay!  I have a finite number of serial-numbered keys and a finite number of staff at any one time.  So keeping a table of 'active' keys and active staff is pretty easy.   What I'd like to do is create a split form, where I open a staff member's record and in the split have a continuous form that allows me to track which key(s) have been issued to that person, with check-out and check-in dates.  That I can manage.  What I'm hoping to do is to ensure that I don't accidentally issue the same key to multiple people.  
Does anyone have any tips or examples that I can reverse-engineer that ensure that a key that is currently issued to PersonA is unavailable to issue to anyone else until it is checked back in?
Once I get that done, I should be able to figure out how to do the same in reverse, by opening up a key's record and seeing the history of who it was checked out/in to.
Thanks!