The ornamental design for a multi-cap duffle bag, as shown and described.
FIG. 1 is a perspective view of a multi-cap duffle bag;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The long broken lines illustrate portions of the multi-cap duffle bag and form no part of the claimed design. The shorter broken lines illustrate stitching and form no part of the claimed design.