Why are there multiple instances of same person on shared workshee
 

We have a shared worksheet on a mapped drive on our server. Every month I
get a work order to find out why no one can get into the spreadsheet. When I
open up the Shared Worksheet window there are multiple instances of the same
person having the worksheet open. These will date back months. I know the
users are closing the worksheet correctly, but why would it allow multiple
instances like this?