I would say that there is an endnote already there but not showing. For example, could it be part of a deleted tracked revision and you aren't showing those deletions.

If you add an endnote at the top of the document, does it number as 1? You could move it progressively down the document until it clicks over to 2 which would show you where the 'hidden' endnote is located.

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Andrew Lockton
Chrysalis Design, Melbourne Australia