Commerce Feeds - Taxonomy term processor Issue
Tom, I found your question today while fighting the same problem in the standard Feeds module.  In the Processor > Mapping setup, do you have fields set as unique targets in the "target configuration" column? 

In my system, I found that even though I had tried to set the imported column to be a 'unique target', that setting had not been saved. After going through the process of setting unique targets, updating the column map, and saving again, the duplication problem has now been solved in my system.