The bill does not seem to have been undone. If it had been undone the entries would not show as billed. 

If you duplicate the entries you will have extra time in the system. If you are measuring productivity then you would have wrong information. 

If you bill the duplicate entries you can control the invoice number used by changing the settings on the individual matter to use a specific invoice number but you will have double AR becase the first bill is still there. 

The bill does not seem to have been undone. If it had been undone the entries would not show as billed. That's what I thought, too, but when I click on Bills/Undo Approved Bills, that bill does not appear in the list.

A bill appearing on the undo list is not indicative of whether it can be undone or not. Unless you are on Timeslips Premium Elite only the last bill for a client can be undone. Also, there are a number of actions that can be taken that can prevent a bill from being undone including things like rebuild AR, running another bill for the matter and putting it in proof stage, etc. 

So I would say, the bill was NOT undone and cannot be undone (unless you have Premium Elite). 

If you have Premium Elite then you may want to contact support for assistance.