I don't think double entries should be accepted even if the user can sign a message to the initial staked address. It sets a precedence that could be used for account sales and can easily be abused. This is of course not saying that's what Husna QA is doing, but such loophole shouldn't exist at all.
Sometimes there are reasons to stop trusing an old address:
In June, 2016, I accidentally copied the private key for 1foreverDArUNEX2gVD26vautcx3b8zTZ in my Google search bar. That's been bugging me ever since. ~ I've downloaded
my data from Google, and it confirms Google still knows the private key. It's not something I worry about that much, but it's a loose end to tie up.
Today is April 18, 2019. I, LoyceV, am "unstaking" address ~ at Bitcointalk.org and staking new address ~.