A requester or technician is unable to self approve using python script

I have a trigger to call a python script that pull an e-mail address from an additional field on an incident template and adds the email address to the approvals list

This in general works really well except

if the logged in technician or requester completing the incident request and also enters their own email address into the additional field (this does occasionally happen when the person completing the incident in also a Line manager with approval), the request is created but there is no approval tab.

Looking in the history of the request that is created there is a comment:-

The following e-mail ID's cannot be included for Request Approval: : [pa**.*** ter@hey.n**.uk]

and yet If the requester/technician opens the request they have just created they are able to add themselves as an approver "using send for approval"

Any ideas what Im missing?