Well, if you tell it to do so.  Matthew Dillon has added a user-settable limit to the amount of memory used during deduplication, so if your Hammer-using system is low on RAM, you can conserve.  This is probably most useful if you are running DragonFly in an extremely small VM, or if your name is Venkatesh.

(inside joke; Venkatesh has a crazy old desktop for DragonFly.)