The new IAGSDC website requires a unique email address for each profile. Since many profiles on the old IAGSDC website shared an email address, that presented a conversion problem.
We initially thought it might be possible to clean up duplicate email addresses before the move to the new site, but the response to that effort has been underwhelming. That probably makes sense; if you created a profile with an email address already in use by someone else, it's likely you had a reason to do so, and don't have much interest in changing.
So . . . what to do?
Since many of the profiles on the old system have registration data attached to them, not moving them was not an option. But moving them with the same duplicate email address was also not an option, and getting people to change their duplicate email was not going to happen. So: we made up fake email addresses. Yes, you read that right. Profiles on the old system that had a duplicate email address were assigned a new, fake email address in order to be able to move them. The fake addresses are in the form of "firstname.lastname@example.org," where the number in the address is different for each duplicate to keep the addresses unique.
This solution enabled us to move the profiles and related registration data, but it does have a downside, in that anyone who has a profile with a fake email address will be unable to use the system to retrieve a new password. We've got the data; you just have no way to access it. This falls into the "solve one problem, create another" pattern of life-hacks, but at least we kept the data. Depending on demand, we'll see if there is some systematic way to address these old profiles when the new system has settled down. Or maybe there will be no systematic way to address the old profiles, and we'll just deal with them on an ad hoc basis if/when people inquire about them.