Okay, what I'm reading into this is that in order to borrow 15 coins (sell 15 repocoins worth of bitcoin with an offer to buy the same amount of bitcoin back for ~15.005 repocoins the following day) the borrower must "lock" collateral worth a bit more than 15 repocoins for the duration of the time he's borrowing. 

If this collateral is repocoins, then obviously his borrowing makes no sense because repocoins are fungible.  whatever purpose he has for the repocoins he intended to borrow would be just as well served by the repocoins he's locking up for the duration.  Therefore I conclude that the collateral here must be in some other asset.  But if the collateral is in Bitcoins, then he's locking up the same amount of Bitcoins that he's loaning out, so wouldn't he be charging additional interest because this is tying up twice as much of his assets as are actually appearing on the market?