DBSlicer

The DBSlicer module allows you to export and import the content of your database.
It uses database slicing collect the rows which will be exported.
With dtabase slicing you can select a smaller piece of the database (this we call slice) that contains all of the records belonging to the slicing criterion. The slicing criterion can be for example a node.
The module needs to know the connections between our database tables. These connections have been defined in the module for the core database tables.
You can import and export the result of a slice in XML format, wich will contain the necessary records.

A possible use case:
You want to export a node from your site and import it into another site. With the DBSlicer module you only have to select the node, you want to export and the module will generate an XML file. The XML file contains all informations about the author, the comments, etc. You can import this slice into another installation of Drupal. During the import process, when the module wants to insert the node, first it check its existence in the database. If it already exists, you can update it. The search does not depend on the primary keys. The primary keys are not stored in the XML file. The module uses the content of the record to find it in the database. With this method there won't be any duplicates in your database after the import.