Improving the onboarding experience for existing app users

If someone already has their own Gainz account and then starts working with a coach through Gainz, the following happens:

Client is created → client receives invitation email with link → clicking the link = user already exists → the client is NOT added to the client overview in Gainz.

When working with people remotely, this causes confusion on both sides. Can the workflow for existing app users be refined in this case?