Edebug doesn't deal well with backtracking out of definitions, see
Bug#41988. Rather than trying to support this rare situation (e.g. by
implementing a multipass parser), prevent it by adding an implicit
gate.
* lisp/emacs-lisp/edebug.el (edebug--match-&-spec-op): Disable
backtracking when hitting a &define keyword.
* test/lisp/emacs-lisp/edebug-tests.el
(edebug-tests-duplicate-&define): New unit test.
(edebug-tests--duplicate-&define): New helper macro.
* doc/lispref/edebug.texi (Backtracking): Mention &define in the list
of constructs that disable backtracking.
* etc/NEWS: Document new behavior.
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