diff -r 0f84e0e6400e app/controllers/issues_controller.rb
--- a/app/controllers/issues_controller.rb	Sun Sep 20 06:20:40 2015 +0000
+++ b/app/controllers/issues_controller.rb	Tue Oct 20 17:14:24 2015 +0200
@@ -411,7 +411,7 @@
         unless User.current.allowed_to?(:copy_issues, @copy_from.project)
           raise ::Unauthorized
         end
-        @link_copy = link_copy?(params[:link_copy]) || request.get?
+        @link_copy = link_copy?(params[:link_copy])
         @copy_attachments = params[:copy_attachments].present? || request.get?
         @copy_subtasks = params[:copy_subtasks].present? || request.get?
         @issue.copy_from(@copy_from, :attachments => @copy_attachments, :subtasks => @copy_subtasks, :link => @link_copy)
diff -r 0f84e0e6400e app/models/issue.rb
--- a/app/models/issue.rb	Sun Sep 20 06:20:40 2015 +0000
+++ b/app/models/issue.rb	Tue Oct 20 17:14:33 2015 +0200
@@ -226,7 +226,7 @@
   # Copies attributes from another issue, arg can be an id or an Issue
   def copy_from(arg, options={})
     issue = arg.is_a?(Issue) ? arg : Issue.visible.find(arg)
-    self.attributes = issue.attributes.dup.except("id", "root_id", "parent_id", "lft", "rgt", "created_on", "updated_on")
+    self.attributes = issue.attributes.dup.except("id", "root_id", options[:link]? "parent_id" : "", "lft", "rgt", "created_on", "updated_on")
     self.custom_field_values = issue.custom_field_values.inject({}) {|h,v| h[v.custom_field_id] = v.value; h}
     self.status = issue.status
     self.author = User.current
diff -r 0f84e0e6400e app/models/issue_relation.rb
--- a/app/models/issue_relation.rb	Sun Sep 20 06:20:40 2015 +0000
+++ b/app/models/issue_relation.rb	Tue Oct 20 17:14:39 2015 +0200
@@ -97,6 +97,9 @@
   def validate_issue_relation
     if issue_from && issue_to
       errors.add :issue_to_id, :invalid if issue_from_id == issue_to_id
+      if issue_to.parent_id && issue_from.parent_id
+        errors.add :issue_to_id, :invalid if issue_from.parent_id == issue_to.parent_id
+      end
       unless issue_from.project_id == issue_to.project_id ||
                 Setting.cross_project_issue_relations?
         errors.add :issue_to_id, :not_same_project