STEP 1:
sort all lines
:sort u
STEP 2:
remove duplicates , the first leaves only the last line, the second leaves only the first line.
g/^\(.*\)$\n\1$/d g/\%(^\1$\n\)\@<=\(.*\)$/d
Breakdown of the second version:
g//d <-- Delete the lines matching the regexp \@<= <-- If the bit following matches, make sure the bit preceding this symbol directly precedes the match \(.*\)$ <-- Match the line into subst register 1 \%( ) <--- Group without placing in a subst register. ^\1$\n <--- Match subst register 1 followed by end of line and the new line between the 2 lines
