RIA Services with DataForms are in the focus of Jeff Handley's article explaining the Add/Remove issues.
It turns out that there’s nothing built into RIA Services to easily work around this problem right now. And we haven’t yet decided if/how we’ll address the issue in later releases. But I came up with a solution that people could use today. It comes with no warranties, no guarantees that it won’t kill your cat or cause you other sorts of pain. But from my limited testing, it seems to work for me.