When an issue is "new" or "open", I get radio buttons for chosing the new issue state. That is great -- I can pick "duplicate" and enter the number of the master issue.
However, I can also set the state via a dropdown menu. That menu duplicates some (all?) of the options and makes little sense: there is no way to indicate the master issue when I choose "duplicate" in the dropdown menu.
I suggest only using radio buttons for this, that seems to give the best overview. The radio buttons should probably always be shown, even for "resolved" or "duplicate" issues.