Entity linker should skip already linked concepts

[reckart]

Is your feature request related to a problem? Please describe.
The entity linker recommender will always propose the same concepts to link to a target candidate. If a layer supports stacking and I have already accepted one of the proposed concepts, this concept does not need to be shown anymore - I do not need duplicates.

Describe the solution you'd like
If a concept has already been linked to an annotation at the same location as the target candidate, do not propose that concept again.

It is quite possible that the recommendation renderer will already suppresses the duplicate concept - needs to be checked.