You need to take note of which items are selected (by name), then find those names in the new ordered list, and select them.

use strict;
use Tk;

use Data::Dumper;

my @data = (qw( ccc eee ddd aaa aaa ccc ddd ddd bbb ddd eee ggg fff));
my %seen;

# Create & configure text widget...
my $top = MainWindow->new;
my $list = $top->Scrolled('Listbox')->
    pack(-side=>'right', -fill=>'both', -expand =>1);

my $ref;
$ref = $top->repeat(700, \&addItem);

sub addItem
{
    my $item = shift(@data);
    if(!$item) { $ref->cancel; return };
    print STDERR "$item\n";
        # Selected items (by name)
        my @selected_items = map {$list->get($_)} $list->curselection;
        print "Selected items: @selected_items\n";
    if(! $seen{$item}) {
        my @new_selection_order = sort keys %seen;
        # Map names to list position
        my %new_selection = map {($new_selection_order[$_] => $_)} 0..
+$#new_selection_order;
       $list->delete(0, 'end');
       $list->insert('end', @new_selection_order);
           # Set selection(s)
           print "New indexes should be @new_selection{@selected_items
+}\n";
           $list->selectionSet($_) for @new_selection{@selected_items}
+;
    }
    $seen{$item}++;
}

MainLoop();
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