Hi,

Either you can use the constrains or override the create/write method to show the warning message. First method will be better.

You can add below code in your function,

existing_task = self.env['project.task'].search([('user_id', '=', self.user_id.id), ('date_field', '=', date_value)])
if existing_task:
    raise ValidationError(_('Message..!'))

This is just a rough code which is not tested, you can set the search condition accordingly and raise the warning.

Also Check this : Constrains in Odoo

Thanks