[FIX] link_tracker: do not allow link tracker duplication

Purpose
=======
We should not be able to duplicate the link trackers (as it
makes no sense from a functionnal POV).

Task-2090344

closes odoo/odoo#50279

X-original-commit: 850e7442
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>