Do not duplicate cells+layers on "Duplicate"

See https://www.klayout.de/forum/discussion/2335 for the rationale

src/laybasic/laybasic/layLayoutViewBase.cc

@@ -5297,11 +5297,7 @@ LayoutViewBase::paste_interactive ()
 
   std::unique_ptr<db::Transaction> trans (new db::Transaction (manager (), tl::to_string (tr ("Paste and move"))));
 
-  {
-    //  let the receivers sort out who is pasting what ..
-    do_paste ();
-    lay::Editables::paste ();
-  }
+  lay::Editables::paste ();
 
   //  temporarily close the transaction and pass to the move service for appending it's own
   //  operations.
@@ -5314,6 +5310,12 @@ LayoutViewBase::paste_interactive ()
 
 void
 LayoutViewBase::copy ()
+{
+  copy_view_objects ();
+}
+
+void
+LayoutViewBase::copy_view_objects ()
 {
   cancel_edits ();
   if (! lay::Editables::has_selection ()) {

src/laybasic/laybasic/layLayoutViewBase.h

@@ -264,6 +264,13 @@ public:
    */
   virtual void copy ();
 
+  /**
+   *  @brief Copies to clipboard (view objects only)
+   *
+   *  This version does not look for copy sources in the tree views.
+   */
+  void copy_view_objects ();
+
   /**
    *  @brief Cuts to clipboard
    *

src/layui/layui/layLayoutViewFunctions.cc

@@ -1177,7 +1177,7 @@ LayoutViewFunctions::do_cm_duplicate (bool interactive)
   db::Clipboard::instance ().swap (saved_clipboard);
 
   try {
-    view ()->copy ();
+    view ()->copy_view_objects ();
     view ()->clear_selection ();
     view ()->cancel ();
     if (interactive) {