diff --git a/lib/Slic3r/Model.pm b/lib/Slic3r/Model.pm
index 2d7436c8b..d9b5ef463 100644
--- a/lib/Slic3r/Model.pm
+++ b/lib/Slic3r/Model.pm
@@ -166,7 +166,8 @@ sub duplicate {
     # note that this will leave the object count unaltered
     
     foreach my $object (@{$self->objects}) {
-        foreach my $instance (@{$object->instances}) {
+        my @instances = @{$object->instances};  # store separately to avoid recursion from add_instance() below
+        foreach my $instance (@instances) {
             foreach my $pos (@positions) {
                 ### $object->add_instance($instance->clone);  if we had clone()
                 $object->add_instance(
diff --git a/lib/Slic3r/Print.pm b/lib/Slic3r/Print.pm
index 2114ead08..458dd2f77 100644
--- a/lib/Slic3r/Print.pm
+++ b/lib/Slic3r/Print.pm
@@ -191,6 +191,7 @@ sub validate {
                 # now we need that no instance of $convex_hull does not intersect any of the previously checked object instances
                 for my $copy (@{$object->_shifted_copies}) {
                     my $p = $convex_hull->clone;
+                    
                     $p->translate(@$copy);
                     if (@{ intersection(\@a, [$p]) }) {
                         die "Some objects are too close; your extruder will collide with them.\n";
@@ -837,14 +838,13 @@ sub write_gcode {
     
     # do all objects for each layer
     if ($Slic3r::Config->complete_objects) {
-        
         # print objects from the smallest to the tallest to avoid collisions
         # when moving onto next object starting point
         my @obj_idx = sort { $self->objects->[$a]->size->[Z] <=> $self->objects->[$b]->size->[Z] } 0..$#{$self->objects};
         
         my $finished_objects = 0;
         for my $obj_idx (@obj_idx) {
-            for my $copy (@{ $self->objects->[$obj_idx]->copies }) {
+            for my $copy (@{ $self->objects->[$obj_idx]->_shifted_copies }) {
                 # move to the origin position for the copy we're going to print.
                 # this happens before Z goes down to layer 0 again, so that 
                 # no collision happens hopefully.
@@ -929,8 +929,7 @@ sub write_gcode {
         # append full config
         print $fh "\n";
         foreach my $opt_key (sort @{$self->config->get_keys}) {
-            next if $Slic3r::Config::Options->{$opt_key}{shortcut};
-            next if $Slic3r::Config::Options->{$opt_key}{gui_only};
+            next if @{$Slic3r::Config::Options->{$opt_key}{shortcut}};
             printf $fh "; %s = %s\n", $opt_key, $self->config->serialize($opt_key);
         }
     }
diff --git a/lib/Slic3r/Test.pm b/lib/Slic3r/Test.pm
index f4f83fed4..3a3944318 100644
--- a/lib/Slic3r/Test.pm
+++ b/lib/Slic3r/Test.pm
@@ -90,7 +90,7 @@ sub model {
     my $object = $model->add_object;
     $object->add_volume(mesh => $mesh);
     $object->add_instance(
-        offset      => Slic3r::Point->new(0,0),
+        offset      => [0,0],
         rotation    => $params{rotation} // 0,
     );
     return $model;
@@ -108,6 +108,9 @@ sub init_print {
     $model_name = [$model_name] if ref($model_name) ne 'ARRAY';
     for my $model (map model($_, %params), @$model_name) {
         die "Unknown model in test" if !defined $model;
+        if (defined $params{duplicate} && $params{duplicate} > 1) {
+            $model->duplicate($params{duplicate} // 1, $config->min_object_distance);
+        }
         $model->arrange_objects($config->min_object_distance);
         $model->center_instances_around_point($config->print_center);
         $print->add_model_object($_) for @{$model->objects};
diff --git a/t/gcode.t b/t/gcode.t
index 17f4ed824..c8db2f816 100644
--- a/t/gcode.t
+++ b/t/gcode.t
@@ -48,13 +48,13 @@ use Slic3r::Test;
     # - no hard-coded "E" are generated
     # - Z moves are correctly generated for both objects
     my $config = Slic3r::Config->new_from_defaults;
+    $config->set('gcode_comments', 1);
     $config->set('complete_objects', 1);
-    $config->set('duplicate', 2);
     $config->set('extrusion_axis', 'A');
     $config->set('start_gcode', '');  # prevent any default extra Z move
     $config->set('layer_height', 0.4);
     $config->set('first_layer_height', 0.4);
-    my $print = Slic3r::Test::init_print('20mm_cube', config => $config);
+    my $print = Slic3r::Test::init_print('20mm_cube', config => $config, duplicate => 2);
     ok my $gcode = Slic3r::Test::gcode($print), "complete_objects";
     my @z_moves = ();
     Slic3r::GCode::Reader->new->parse($gcode, sub {
diff --git a/t/retraction.t b/t/retraction.t
index 8221c4deb..636359149 100644
--- a/t/retraction.t
+++ b/t/retraction.t
@@ -11,12 +11,13 @@ use Slic3r;
 use Slic3r::Test qw(_eq);
 
 my $config = Slic3r::Config->new_from_defaults;
+my $duplicate = 1;
 
 my $test = sub {
     my ($conf) = @_;
     $conf ||= $config;
     
-    my $print = Slic3r::Test::init_print('20mm_cube', config => $conf);
+    my $print = Slic3r::Test::init_print('20mm_cube', config => $conf, duplicate => $duplicate);
     
     my $tool = 0;
     my @toolchange_count = (); # track first usages so that we don't expect retract_length_toolchange when extruders are used for the first time
@@ -115,13 +116,13 @@ my $retract_tests = sub {
 
 $retract_tests->('');
 
-$config->set('duplicate', 2);
+$duplicate = 2;
 $retract_tests->(' (duplicate)');
 
 $config->set('g0', 1);
 $retract_tests->(' (G0 and duplicate)');
 
-$config->set('duplicate', 1);
+$duplicate = 1;
 $config->set('g0', 0);
 $config->set('infill_extruder', 2);
 $config->set('skirts', 4);