Ensure that they are no duplicates in Odoo

[teolemon]

What

• Ensure that they are no duplicates in Odoo
• There's a process to de-duplicate but it is "time consuming" according to Manon. It does not seem to be very smart, since I assume there are no real conflicts in 95% of cases
• There's the non-0 possibility that those are created by automation (eg a person signing up to multiple teams, or a person filling the volunteer form several time)
• There's the other possibility that we import contacts several time (using CSV/XLS files ?) and that Odoo does not attempt/propose to deduplicate at import time

Part of