Add `sly-mrepl-prevent-duplicate-history' to REPL

* NEWS.md (mREPL enhancements): Mention `sly-mrepl-prevent-duplicate-history'

* contrib/sly-mrepl.el (sly-mrepl-prevent-duplicate-history): New
customization variable.
(sly-mrepl--merge-and-save-history): Respect it.