corporate-wage-slave

This is pretty sharp practice if this is the case. What may have happened is that lastminute.com has internally bolted two single trips together. But from your point of view I guess you bought it as a return. So I would either insist on lastminute refunding the full amount, and/or approach your credit card company to see if they will chargeback. Now for the latter you may need to at least inform them sooner rather than later, in case there are timelines involved, but consumer legislation should cover you, again assuming your bought the ticket as a return. Under any reading of consumer protection, what you bought is not something that could be feasibly used.