[FIX] crash when a mode appears twice in view_mode

If an action has for instance twice "tree" in view_mode, you get a crash
when displaying the action.

This patch adds a python constraint on the model which will prevent such
change.

closes odoo/odoo#50189

Opw: #2241415
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>