You can only have one footnote with a given reference number on a page. For your purposes, you might do better to use endnotes and, for cases referring to the same endnote #, insert a cross-reference to that #. That said, such a cross-reference will take you back to the original endnote reference, not to the endnote. If you want the links to go to the actual endnotes instead, that would bookmarking the endnote references in both the body and the endnotes, then using hyperlinks in place of the standard references (which get formatted as hidden text). The process is quite involved and requires a macro to automate it.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]