1. Go to -> Control Panel -> Sites -> Add -> Blank Site. Name: OpenSite1, Membership Type: Open
2. Go to -> My Private Pages
3. Check your My Sites portlet. There is the OpenSite1.
4. Click on Leave next to OpenSite1.
5. Click on Available Sites.
6. Find the OpenSite1, and click on Join.
The next messages appear on the screen: "Your request completed successfully. ", "My Sites is temporarily unavailable. "
In the log a DuplicateKeyException created. See in the attachment.
In Control Panel, I can Leave and then Join (via Manage Memberships) from / to this site without any problem.