If you have the same stock listed in your online shop as you have in your real shop it is possible for an item to be sold in the shop and then be sold again online before you have had a chance to change the online inventory.
A very tricky situation with no tidy answer.
It's easy to imagine that happening, but surely the answer is to be honest and tell
one of the buyers that there's been an error (I realise it might be difficult to determine who got there first).
To go through with the charade of "sending" the item then saying it's been "lost" - as they
may have done here - is just ridiculous.