What you're describing is generally called a "Back2Back" or "B2B". A "Double Dip" cruise normally refers to a single cruise that has two stops at Castaway Cay as part of the same cruise. For some reason, these have become very desirable and usually cost a little more for the same number of nights as a cruise that only stops there once.
Yep, lots of people will prefer to do a 3 day/4 day B2B cruise on the Dream rather than a 7 day on the Fantasy. You get the same number of nights and can even keep the same stateroom if you're lucky. Depending on the season, you can get a great B2B deal that beats a 7 day fare.
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