They'll recalculate it at the one-way rate and then refund if any. Also some tickets that are "refundable" don't allow refunds once the outbound portion has been flown. Regardless though, what you intend to do won't work out well. It's against the Contract of Carriage, and on top of not getting money back, you may;
1)Be demanded to pay the extra amount - with possible legal action if you refuse
2)Have your frequent flyer account closed and all miles and any existing awards canceled.

DL like all airlines takes it vary seriously any attempts to get around paying the correct fare for your trip.

Bottom line, do it once, and don't ask for a refund, and nothing will normally happen to you. Ask for a refund, and you're asking for DL to come at you.