$user_data = "
<table cellpadding=\"0\" cellspacing=\"0\">
<thead>
 <tr>
  <td colspan=\"4\">User List</td>
 </tr>
</thead>";

my %seen;
for (my $i = 0; $i < @$sql_data; $i++) {
    my $user = $sql_data->[$i]{user};
    
    if ($seen{$user}) { # <-- added flow control
        next;
    }
    $seen{$user} = 1;
    
    my $link = "<a href=\"goto.pl?user=$user\">$user</a>";
    my $first = $sql_data->[$i]{first};
    my $last = $sql_data->[$i]{last};
    my $phone = $sql_data->[$i]{phone};
    my $mail = $sql_data->[$i]{mail};
                                
    $user_data = "$user_data<tr><td><div>$show</div></td><td><div>$last, $first</div></td><td>$phone</td><td><div>$email</div></td></tr>";
}
$user_data = "$user_data</table>";