Not really sure how to replicate it... I've tried double clicking follow really fast, etc. and I know there's code that checks if they are already following before it tries to follow them, but for whatever reason there's a case where that check fails when it shouldn't somehow.
We aren't using any custom database adapter, no master/slave setup, no custom datawriters or models that would affect it.
Maybe a good idea for it to be an INSERT IGNORE in this case?