If you merely want to check for duplicates under the galleries folder and get a list of files (which you then can work with), save the following code as a duplicates.sh shell script and give it the execute bit and invoke it in your Piwigo folder (above the galleries folder). Ensure you keep and don't omit the two apostrophe / single quote characters that enclose the awk script when copy-pasting the code.

#!/bin/sh
find galleries -type f -print0 | xargs -0 md5sum | awk '
    {
        ++s[$1];
        f[$1][s[$1]] = $2
    }
    END {
        for (x in s) {
            if (s[x] > 1) {
                for (i in f[x])
                    print x, f[x][i]
            }
        }
    }
'

This script can be adjusted to also check files under the upload directory by simply adding that to the list of find directories, as in find galleries upload -type f ...

This needs gawk for true multidimensional arrays. If you happen to have only mawk or another inferior awk implementation you're out of luck and need to rewrite.