[FIX] calendar: multiple notifications for an event

Before this commit, when an event have multiples notifications, if the
first notification is not ack when the second notification is shown, not
only two are visualized but multiples notifications are raised.

Now, only the correct number of notifications are shown.

opw-2070852

closes odoo/odoo#39807

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>
Co-authored-by: Aaron Bohy <aab@odoo.com>
Co-authored-by: Lucas Perais <lpe@odoo.com>