I can't grant it to anybody, I can't destroy it, I can't win or lose the War of the Prussian Revolt as that has properly ended, nor can I usurp, revoke or destroy the legitimate Duchy of Prussia to see it that would help. Suddenly, most of the Diplomatic actions aren't available to me anymore, as one of the conditions is 'Emperor Echmarcach the Great must not currently hold a Temporary title' and there doesn't seem to be a way to get rid of it. But then the latter apparently died a natural death and I inherited the 'Prussian Revolt' title from him.
However, I think I've just run into another bug, one that isn't so easy to ignore: One of my vassals is the Duke of Prussia and one of his vassals was rebelling against him. With all the crusading I'm doing and holy warring I'm planning to do, I think I can live with that Antipope and will leave it as it is for now.
The Swedish crown has been through some troubled times in my game, so I guess there was a lot of opportunity for that glitch to occur.
