When a user submits survey via Email New Assessment Instance is created rather than updating exisintg Assessment instance
This issue is occuring because the associated notification has a mail script defined which is creating a new Survey instance rather than updating the existing survey instance when submitting a Survey via Email.
We suggest referencing the logic implemented in OOB (Assessment: Survey User Invite) notification below which queries the correct survey instance for the Requestor:
var link = new AssessmentUtils().getAssessmentInstanceURL(current.sys_id);
var url = '' + link + '';