>perl -wMstrict -le
"use List::MoreUtils qw(uniq);
 ;;
 my @seqs = qw(AGCT AGGT GG AGCT CTAG);
 ;;
 my $seen  = '';
 my $delim = ':';
 ;;
 my @no_dups =
   grep { ($seen !~ m{$_}xms) && ($seen .= $delim . $_) }
   uniq
   @seqs
   ;
 print qq{'$_'} for @no_dups;
"
'AGCT'
'AGGT'
'CTAG'