I did, and I ended up getting charged for the journey twice, but could not get a refund as the total of the unused tickets were under ^10. The next journey I'm making only cost ^4 anyway.
Did you select the wrong number of tickets when making the booking, or did
FGW▸ make the mistake? If it's the latter then FGW legally would have to fully refund the ticket(s) that you didn't order, regardless of any administration charges that are specified if you request a refund for a ticket that you no longer require.