This error seems to be caused by Fortress listing the same file in the changeset multiple times. I'm not sure what prompts it to record two separate instances in the changeset, but I have to "undo" one of them using the client before I'm able to check in my changeset. Just wanted to bring this up - the work-around isn't too complicated and it doesn't happen often, but renaming a file in VS that already has a pending change seems to confuse Fortress.An item with the same key has already been added.
I'm currently using 2.0.4.18845 for both the server (64-bit) and client.