A double-entry visa usually applies in the same manner as a single-entry visa. The only difference between a single-entry visa and a double-entry visa is that the second one gives you the option to return after you have left the Schengen area again.
You should be very careful not to exceed the number of days allowed to remain in the Schengen Zone, as well as the time during which you are allowed to stay in the Schengen Zone.