The technical term is a cross-reference, here is Help's answer:

"Refer to the same footnote or endnote more than once
1. Click where you want the reference located.
2. On the Insert menu, click Cross-reference.
3. In the Reference type box, click Footnote or Endnote.
4. In the For which box, click the note you want to refer to.
5. In the Insert reference to box, click Footnote number or Endnote number.
6. Click Insert, and then click Close.
Notes
The new number that Word inserts is actually a cross-reference to the
original reference mark. If you add, delete, or move a note, Word updates
the cross-reference number when you print the document or when you select
the cross-reference number and then press F9. If you have trouble selecting
the cross-reference number by itself, try selecting some surrounding text
along with it, and then press F9.
If you want a footnote or endnote cross-reference to appear in superscript
text, select Footnote number (formatted) or Endnote number (formatted)."

IOW, you have to manually order it to renumber by hitting F9 to update
fields, it will not automatically change when you insert a new note, as
regular notes do.

DM