I think I'll report this as a bug. It may be working as designed but I think it may need changing! On my site, for various reasons we have on occasion allowed members to sign up a second account with the same email address as the first, meaning more than one account exists with the same email address. On importing users into XenForo, the importer highlights this, and gives only two options for each user: merge the accounts, or change one of the email addresses so they are all unique. To get past this I had to insert a random character into the duplicate email addresses and then manually remove them after import. There should be a "skip" option at this stage for if you simply want to leave things exactly as they are!