You are correct that it is sometimes possible to earn miles twice this way. You will not forfeit your miles on the original card.

But it only works when miles are offered for balance transfers - which is usually not the case.

However, if you have a good enough offer for a balance transfer, this might be a good idea.

My personal experience is that balance transfer offers that earn miles aren't as good as balance transfers without miles.

IE, My United Card offered me 5.9% fixed APR on balane transfers + up to 2,500 miles per transfer check. But I have received offers in the past from them for 2.9% fixed APR without the miles on balance transfers.