Allow tag error if error already exists in reference

When xliff files from third party applications are not well formed and contain duplicated tags. Then it is not possible to finish a task.

Need to fix import so that if the same tag already exists we have to append a "-duplicate-COUNT" to its ->id property, where COUNT should count on each duplicated tag separately. That should ensure that if a tag is duplicated in the source also, the target duplicates get the same tag nr as in the source.