There is an existing FORUM question https://community.norton.com/en/forums/warning-username-already-exists from May 2015. That question has closed the replies option. However, a solution is still needed. This may be a solution -- use the "*" character in the URL.
I am on the faculty of a local university. They have simplified sign-in so that I can use the same USERID and PASSWORD on sites within the university -- their Advisor sign-in site (https://banner.stthomas.edu/), their Blackboard class site (https://blackboard.stthomas.edu/), their OUTLOOK site (https://mail.stthomas.edu/), etc. The URLs for those various sites are similar but different. Norton Identity safe makes me choose which single URL I want to access with Norton Identity Safe. For the other URL sites that would otherwise accept the same USERID and PASSWORD, Identity Safe doesn't fill in any boxes so I must manually enter it. If I try to create a NEW LOGIN on Identify Safe, with a different URL but the same USERID and PASSWORD I receive this message: "WARNING! The provided username already exists for this site. Please use a different username and try again."
After much trial and error, I found this fix. Create a NEW LOGIN and use this for the URL, https://*.stthomas.edu Hope this helps others with similar issues.
JVF_MN