in Excel, it can probably be done, but it really depends on how the spreadsheet is made up.
I can't quite get my head round it from the example you have given.
It may be that a better way it to redesign the Excel sheet, (assumming it does already exist?), such that you can't actually make a double booking.
However, though I am not an expert in Access, (or any database), my gut reaction is that Access is probably a better tool for this task, so I'll leave it to others!
If the Excel sheet does already exist, if you can provide a better description of the layou I'll have a think.