Re: avoid duplicates in history

[Richard Stallman]

    Meanwhile, I'd like to propose a related feature.  It would be useful
    to be able to delete some old history elements from the history list.
    I think a good key binding for this command in the minibuffer is M-k.

I think that is too obscure to be included in Emacs;
the code is short, but the cost in conceptual complexity
is too great.  So please don't install it.

Those who would like it can put it in their .emacs files.