Bug 20148: Prevent adding same user multiple times to acq basket

Bug 20148: Prevent adding same user multiple times to acq basket

When adding a user to manage acquisitions basket, it is possible
to add the same user multiple times. Prevent that.

Test plan:

1) Go to Home -> Acquisitions -> [bookseller] -> [basket]
2) On the "Add user" in the Managed by-part, and try adding
   the same user multiple times.
3) Apply patch
4) Repeat 1 and 2. You should now get an error message saying
   the user is already in the list.

Signed-off-by: Pasi Kallinen <pasi.kallinen@joensuu.fi>
Signed-off-by: Roch D'Amour <roch.damour@inlibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>