The error message originates from Adobe and indicates that the user attempting to be created already exists in Adobe. Typically, our service can catch that - this is done via the matching attributes in the provisioning configuration. It may be that there are multiple attributes that are all considered for uniqueness - i.e.: SCIM "userName", "emails[type eq "work"].value, etc, and that the configuration in the Adobe app may only be looking for a match between userPrincipalName -> userName, and not checking for a second attribute like mail -> emails[type eq "work"].value.
First place to start would be talking to Adobe's support to get them to explain why they're returning this error, and from there you should be able to work back what the issue is with the Azure AD provisioning config, if there is one.