In a physical book footnotes and endnotes can be referred to in multiple places in the text without problem as the reader will have a thumb jammed in the pages to be returned to.

Since e-books don't have this elegant bookmark system it might be best to abandon what works in the physical world and just duplicate the information in multiple endnotes each linking back to their own spot.

The other option is to not duplicate the endnote symbol in the text but assign unique identifiers and double those identifiers in the endnotes.

eg.

This is a test fedunster[1]. It is only a test fedunster[2]. If it was a real sprool[3] you would have been notified.

[1] [2] fedunster: a made up word that describes anything that looks like it might have a useful purpose but whose name is not known.

[3] sprool: a made up word that describes the sound a cassette tape shell makes unwinding as you hurl it down a hallway while holding onto a bit of the tape ribbon.