When prefixing or suffixing list variables, check that the value or
values aren't already part of the list. If this is the case when
suffixing, the list won't be touched at all. When prefixing, however,
the last matching instance of the value will be moved to the beginning
of the list. Any remaining duplicates of the value will be left as-is.
Co-authored-by: Vincenzo Mantova <xworld21@users.sf.net>
