That's correct if you have your character in play it restricts you from banding to your opponents version of that unique character
This also means if you have a green David you can't band to your opponents king David
At least that's how I've been told it works
Yes, completely correct. To clarify how this is different from captured characters (as this is a spin-off from the other thread), it was ruled
that captured characters are not characters for the purpose of duplicates. However, you still may not control two copies of the same unique character (even if they have different names, but are the same character) at the same time, including with a band.
To clarify, the rules state now that all cards who are the same character as one you control are
protected from all abilities that would bring them under your control. There are very, very few cases where you can end up with duplicates anymore because of this, and at that point the 'original' (one already controlled by the player now controlling duplicates) stays and all others are discarded.