Duplicate node on content creation Each time any node is being created, Drupal creates two copies of the same node, both in the default language. 

What can be the reason of this duplication?
This is not the normal Drupal behavior.  In this case, what you need to do is start disabling modules one at a time.

Disable a module that you think might be causing the problem (just take a guess) and then create a piece of content to see if two pieces of content are created.  If you disable a module and then create content and you only get one piece of content, you know you have found the problem module.

Once you find the problem module, try to locate the source of the problem in the module configuration.  If you can't find it yourself, try asking again on this site.

Without knowing what module is causing the problem, it is very hard to answer your question, as there are too many potential modules that can cause problems.