The ornamental design for a double fanblade, as shown and described.
FIG. 1 is a side perspective of a double fan blade showing my new design;
FIG. 2 is a side perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a side elevation view thereof; and,
FIG. 6 is a side perspective view thereof.
The broken lines in drawing FIGS. 1-2 depict environmental subject matter. The broken lines in drawing FIGS. 3-6 illustrate portions of the double fan blade that form no part of the claimed design.