#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11107128
use warnings;

my @servers = (
    { Host => 'server02', GroupName => 'serversA' },
    { Host => 'server04', GroupName => 'serversA' },
    { Host => 'server02', GroupName => 'serversB' },
    { Host => 'server04', GroupName => 'serversB' },
    { Host => 'server02', GroupName => 'serversC' },
    { Host => 'server04', GroupName => 'serversC' },
    { Host => 'server02', GroupName => 'serversD' },
);

local $" = ' is in Groups ';
$_ = join '', sort map "@{[ @$_{qw(Host GroupName)} ]}\n", @servers;
1 while s/^(.+$").*\K\n\1/,/m;
print;
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Outputs:

server02 is in Groups serversA,serversB,serversC,serversD
server04 is in Groups serversA,serversB,serversC
[download]