I have some applications installed multiple times, one version in /usr another
one in /opt/gnome etc.  I also have two versions of evolution that have
installed .desktop files in $prefix/share/applications and $prefix/share/applnk

As a result I now have many duplicates showing in the menus.  This would appear
to be because two separate .desktop files are considered different even if the
contents are identical.

There should be only a single entry for a given Exec-line in a menu, or
Name,Exec pair if that would be less surprising.