I have a site with 3 languages: English, French and Dutch. The tags vocabulary multilingual settings is " Per language terms".

When I translate multiple nodes with the same (untranslated) tags with a translation service, the TM module creates 2 translations for the same tag.
In my tests, this only occurred for 1 language, in my case that was Dutch. The 2 translations are created for the first 2 nodes that get translated, subsequent nodes with the same tags are linked to the second translated tag.

I have tested this with the Google translation and my own translation service.