Hi,

If you need to do this from code side, you can add a constrain and check for existing records and then raise a validation error if existing matches found.

You can achieve it by adding the below code in the corresponding model:

@api.constrains('email')
def _check_email_uniqueness(self):
    for partner in self:
        if partner.email:
            existing_partner = self.env['res.partner'].search([
                ('email', '=', partner.email),
                ('id', '!=', partner.id)
            ])
            if existing_partner:
                raise UserError('Email already exists!')

If you need to achieve it from user interface, you can achieve it using, automated action.

See a similar example depicted here: Prevent Product Duplication without code using automated actions

Thanks