The ornamental design for the duck decoy bag, as shown and described.
FIG. 1 is a perspective view taken from the right front corner and with an attached strap alongside;
FIG. 2, a left side elevation view, the right side being a mirror image thereof;
FIG. 3, a rear elevation view;
FIG. 4, a bottom plan view; and,
FIG. 5, a front elevation view with the strap passing beneath, the rear elevation view being a mirror image thereof.
The broken lines showing portions of the duck decoy bag are included for the purpose of illustrating seams stitched to connect fabric components of the duck decoy bag.