list__merge_and_remove_dups

[Christopher Rodd SPEIRS]

I was looking through list.m, and noticed a bug in
list__merge_and_remove_dups.  I am not sure what the correct behaviour should
be, but the current behaviour seems wrong.  

Currently, list__merge_and_remove_dups runs as follows:
list__merge_and_remove_dups([1,1,1], [1,1,2], [1,1,2]).
list__merge_and_remove_dups([1,1,2], [1,1,1], [1,1,1,2]).