JPEG XL, HEIC / HEIF and AVIF Support would be great
Alldup is a 32 bit program which needs an old 32bit library to handle pictures (FreeImage 3.18 from 2018!).
I think this library support is important to open and compare pictures.
It does not support newer picture formats (HEIF, AVIF, JPEG XL, etc.).
The last update of FreeImage 3.18 is from 2018 and newer versions are not available
or in development. I can not see anything.
So I think it is very difficult to get support for newer files
or you must change big parts of the AllDup sourcecode.
Maybe the developers of FreeImage can add these file formats in the future.
Most of the newer picture libraries are 64bit only.