In endless mode, A regular Duplicate Present works normally and cannot be duplicated. But this is not the case for mystery (yellow question mark box) Duplicate Present, which can be targetted by Duplicate Present.

This leads to infinite presents, as an amped mystery Duplicate Present can duplicate a copy of itself twice.

To repro:

1. Obtain an amped mystery Duplicate Present

2. Open a different instance of duplicate present (mystery or regular, amped or not)

3. Copy your amped mystery Duplicate Present

4. Open your amped mystery Duplicate Present and repeat step 3 ad infinitum

Expected result: On step 3, mystery duplicate present is not listed as an option for duplication

Actual: mystery Duplicate Presents are fair game

edit: grammar, clarity