Is this a bug or a feature? I cannot link/add a doc in more than one entity’s “Documents” collection - when I do, it simply disappears from the last entity where it was previously linked. 
This is very unintuitive - different from adding every other type of entity to a collection.
This is important for market research that impacts more than one entity.
So far this is the one-many relation, which means a Document can have a single parent. As a workaround you can mention a document in text, but overall Documents section will be removed when we’ll implement Bocks (and it will be possible to include Document as blocks into many places)