to this problem: http://nabble.documentfoundation.org/Generate-Unique-List-from-Values-in-Column-td4077250.html
this is the solution in MS Excel,
and to do this in LibreOffice the single difference you have to make is to not click and drag, but to ctrl+click and drag down the cell contents!
so, step by step:
you have a header: row 1.
your list starts in row 2 in column A.
your unique list will start in row 2 column B.
you type =INDEX($A$2:$A$20, MATCH(0, COUNTIF($B$1:B1, $A$2:$A$20), 0)) in cell B2 and hit CTRL+SHIFT+ENTER
this should result in you having one of the values of your A2:A20 list in B2.
now press CRTL and click and drag down the bottom-right corner of B2 cell.
if you drag down long enough, #N/A cell contents should show up on the bottom of your unique list.