Removing duplicates in comma lists

[blefloch]

I'm just recording the bug now, it opens up a whole can of worms about commas in comma-list items, which I don't want to worry about any time soon.

The \clist_remove_duplicates:N function does not treat correctly items with commas (or leading/trailing spaces, but the bug is then more subtle). It "flattens" the list somewhat, turning these items-with-commas into multiple items:

\clist_set:Nn \l_tmpa_clist { a , {a,b,c,d} , b }
\iow_term:x { length:~\clist_count:N \l_tmpa_clist }
\clist_show:N \l_tmpa_clist 
%
\clist_remove_duplicates:N \l_tmpa_clist
\iow_term:x { length:~\clist_count:N \l_tmpa_clist }
\clist_show:N \l_tmpa_clist

gives a first list of length 3 with items {a}, {a,b,c,d}, and {b}, and a second one with length 5 with items {a}, {a}, {b}, {c}, {d}.