There were undocumented changes in xtool 0.6.5, --dedup can be used as -dd so you are able to pass this via freearc provided your arc.ini looks like this
Code:
[External compressor:xtool]
header = 0
packcmd = xtool.exe precomp { -option} -c32mb -t100p - - <stdin> <stdout>
unpackcmd = xtool.exe decode -t100p - - <stdin> <stdout>
then your method will be something like -mxtool:mzlib:
dd