follow instructions on this page
http://www.oooauthors.org/groups/members/howtos/references/repeatedreferences
or :

create a footnote;

Place the cursor where you want the (repeated) reference number.
Click on Insert > Cross-references... from the main menu.
Choose the References tab
Click on Footnotes, in the left section
Click on the reference number you want, in the middle section
Click on the Reference format, in the right section
Click on the Insert button at the bottom of the dialogue box.
Click on the Close button.

cross-reference should also be superscript or whatever formatting was original. 
so that the last step is not required :

Highlight the number which has been inserted automatically, and format it (e.g.) 
as superscript.

summary : automatic formatting of cross-references according to original