org-export: Avoid duplicates in `org-export-collect-footnote-definitions'

org-export: Avoid duplicates in `org-export-collect-footnote-definitions'

* contrib/lisp/org-element.el (org-element-map): New optional argument
  to avoid recursion into certain recursive types.
* contrib/lisp/org-export.el (org-export-footnote-first-reference-p,
  org-export-get-footnote-number,
  org-export-collect-footnote-definitions): Use new argument from
  `org-element-map'.
* testing/lisp/test-org-export.el: Add test.

The new argument allows to force entering footnotes definitions at
a certain time (when their first reference is found) but not a second
time when they are encountered in the parse tree.

Thanks to Jambunathan for reporting this.