I claim the ornamental design for a double flask, as shown and described.
FIG. 1 is a front perspective view of a double flask showing my new design, the rear perspective view rotated 180 degrees being a mirror image thereof;
FIG. 2 is a right side elevation view thereof; the left side elevation view being a mirror image thereof;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a bottom plan view thereof.
The double flask in which my design is embodied can comprise a pair of the flasks shown in my copending design patent application Ser. No. 29/234,728 filed Jul. 13, 2005, the entire disclosure of which is hereby incorporated herein by this reference thereto.