This patch avoids savehist saving symbols duplicated between
'savehist-minibuffer-history-variables' and
'savehist-additional-variables'. I've accumulated symbols on
'savehist-additional-variables' that wind up, over time, in the master
minibuffer list documented as being dynamic.

  The contents of this variable is built while Emacs is running,
  and saved along with minibuffer history.

savehist had no defensive mechanism until now. The patch should do no harm
to users and will transparently save some resources.

TIA,

-Stephane