I'm at my cousins condo and we were playing SSE and wanted to do
online but we had to register, so since I had a Stardock.net account so
I registered his game first,
As it's always been, you can only register any serial on one account,
and they cannot be deregistered or transferred. As this was an accident
though, you should contact support@stardock.com and present your case
to them.
For the record though, only one person can play on ICO with a given serial at any time anyway.