Adding documents to a meeting can be done in three different ways: "Add new document from your computer", "Add existing meeting document" or "Add existing library document". If a document has already been uploaded to a past meeting, make sure to select option 2 (Meeting document), then browse in the list of past meetings and select the desired document. Do the same if the document already exists in your corporate library.
A normal use case is a commission preparing a subject and the Board of Directors approving the same some days later. Members of the commission which will find the same document in the BoD meeting again, probably expect to find the same situation as they had at the end of the preparation, therefore including their individual notes made on the document. By re-using the same document from the previous meeting, you assure to have this. Not only the function copies the basic document, but it also copies all individual versions of it, including the personal annotations of the members.