Bug 22002: (bug 21621 follow-up) Re-add GROUP BY in letter.pl and fix it

Bug 22002: (bug 21621 follow-up) Re-add GROUP BY in letter.pl and fix it

Caused by
  commit 0cffb0f7589aa52d65bda9a8a4934790a141f3a0
  Bug 21621: Remove incorrect GROUP BY from tools/letter.pl
The test plan was not complete enough.

Test plan:
1) Go to Home › Tools › Notices & slips
2) Choose a letter to edit, e.g. PREDUE. Note that there is only a
single row.
3) Click the edit button on the notice.
4) Expand the 'Print' section. Put text into the Message subject and
Message body text areas.
5) Click save, this will take you back to the 'Notices & slips' page.
6) Clone PREDUE for a specific library
7) The 'all libraries' option must list the templates for all libraries,
but without duplicate
8) Select a library in the dropdown list: only the templates for the
given library is listed, without duplicate

Signed-off-by: Barton Chittenden <barton@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>