makeroot.sh: allow duplicate entries even with -f <filelist>
makefs disallows duplicate entries unless the -D option is specified.
Previously makeroot.sh enabled -D unless a filelist was provided via
the -f options. The filelist logic creates an mtree manifest from the
METALOG and the provided filelist by passing them through `sort -u`,
so duplicates were not expected. However, duplicates can still occur
when a directory appears in multiple packages -- for example,