Customer accounts that already exist are still sent an invite email when inviting them to a second service desk

Steps to reproduce

1. Create two Service Desk projects.
2. Invite a customer to the first one, via the "Invite Customers" button on the Customers section of the service desk.
3. Follow the invite link that's sent and sign up for an account.
4. On the second service desk, send an invite to the same account.

The email from the second service desk is sent as normal, but when the user follows the link they get a "Token is either expired or not valid" error on the resulting page.

If a customer already exists in a different service desk, there should be some popup that notifies the user that this is the case, adds them to the service desk as normal, and sends a different notification to the customer to let them know they now have access to the new one (as opposed to sending another signup link).