The ornamental design for a duckbillmask, as shown and described.
1. Duckbill mask
1.1: Perspective
1.2: Front
1.3: Back
1.4: Right
1.5: Left
1.6: Top
1.7: Bottom
1.8: Perspective view in an environment of use
A portion of the front face of the mask is transparent; in Fig. 1.8, the broken lines depict a human head that forms no part of the claim; the shading lines in the reproductions represent contours only and do not illustrate an ornamentation or decoration on the surface of the product.