Follow the next scenario:
1/ create a document, with a dashboard in it and add a few gadgets (they will be objects in the document)
2/ export it
3/ go ahead and continue editing the document, add a few more gadgets, remove the gadgets that you had before the export, add some tags if you want (or any other object for that matter)
4/ import the xar that you exported earlier. Import with "Add a new version to the existing document" and "Import as backup package" options.
The result will be a document that has all the objects inside: the ones that were left in the document after the editing at step 3/ and the ones that were in the xar.
You can test with the attached xar, import it, edit the objects in the document, reimport it.
Note that on my local instance (XEM 3.1.1) I also did a restart of the instance, fearing that the objects are kept in the document cache, but it didn't change anything.