I understand what short interest is (% of shares borrowed but not covered yet). My understanding is that once a share is borrowed, it can't be borrowed again (legally, anyway).
This would lead me to believe that "days to cover" would need to only look at the volume of NON-already-borrowed shares, not of total shares.
Suppose the float of stock XYZ is 100 shares.
Firm A shorts 1 share and the short interest is now 1%. To do this, Firm A borrows this 1 share from Bob and sells it to Nate.
Bob still think he owns this 1 share and he sells it to Alice.
Firm A now decides to close their position by buying 1 share of stock. They CAN'T (as far as I understand) buy it from Alice because Alice technically has 0 shares and is owed 1 share so that makes the available shares able to to be bought back 99 instead of 100.
So how come days-to-cover does not take that into account, or what am I misunderstanding?