Perhaps the issue there is that in this scenario (I have a share; you borrow it from me and sell it to me; then you boroow it from me once more and sell it to me again) currently it's treated as if I own three shares (though two of them have marks on them that they can't be lent).

Perhaps it should be explicitly treated as if I have one share, and two "claims on shares" which are likely to materialize, but are not guaranteed to do so - to make that distinction clear, to ensure that the lending for a short actually involves the lender temporarily giving up that share instead of just marking it as "used".