If you are importing a file which is NOT your default language (i.e. a translation file) then do not include any pre-defined features (i.e. customizedvalue is 0) as this will create a new duplicate feature. The translations for pre-defined features should be done once in the backend via Catalog > Features. For non-default languages add only customized value features.
Also written with the precious contribution of Fadi H. Rezq