The data represents a graph in edge list format, i.e. each line is an edge of the graph specifying the two nodes that are connected. What you call clusters seems to be the connected components of this graph. So read the data into a graph structure then extract the connected components, e.g. in R with the igraph package, something like this (untested):

edge.list <- as.matrix(read.table("edge_list.txt",...)) # read the file as appropriate, turn data into a two-column matrix for use by igraph
G <- graph_from_edge_list(edge.list, directed = FALSE)
clusters <- components(G)

In pseudo code, if you want to try to write it

Initialize an 2D array : 2Darray

Import your file in a dataframe

Sort the dataframe by column one and column two, to get something like :

AA BB

BB AA

BB CC

CC DD

For each line of your dataframe

• If it's the first line of the datafame, create and array and append first element and second element of the line
• Else, does the first element exist in array ?
  • If yes, append array with the second element
  • Else, append 2Darray with array, reinitilize array, append array with first element and second element

At the end in 2Darray you will have your clusters.

Untested

You can use perl or python to get that:

#!/usr/bin/perl

use strict;
use warnings;

my %g = ();
my $a = undef;
my $b = undef;
my $net = 0;
my $fst = 1;

while (<>) {
  chomp;
  ($a, $b) = split (/\s+/, $_);
  if ($fst == 1) {
    #warn "first iteration, adding $a - $b in net $net\n";
    $g{$net}{$a} = 1;
    $g{$net}{$b} = 1;
    $fst = 0;
  }
  else {
    my $new = 1;
    for (my $n = 0; $n <= $net; $n++) {
      if (defined $g{$n}{$a}) {
        if (defined $g{$n}{$b}) {
          #warn "$a - $b exist in net $n\n";
          $new = 0;
          last;
        }
        else {
          #warn "$a exist in net $n, adding $b\n";
          $g{$n}{$b} = 1;
          $new = 0;
          last;
        }
      }
      else {
        if (defined $g{$n}{$b}) {
          #warn "$b exist in net $n, adding $a\n";
          $g{$n}{$a} = 1;
          $new = 0;
          last;
        }
      }
    }
    if ($new == 1) {
      $net++;
      #warn "$a - $b not seen in other nets, adding in a new net $net\n";
      $g{$net}{$a} = 1;
      $g{$net}{$b} = 1;
    }
  }
}

#warn "writting nets\n";
foreach $net (sort keys %g) {
  print join "\t", sort keys %{ $g{$net} };
  print "\n";
}

Example:

$ perl graph.pl < data.txt
AA      BB      CC      DD
EE      FF      GG      HH
II      JJ      KK