Can I see how many duplicates there are of a post? I.e. how many posts have been labelled as duplicate because of this post? If so, how? And can I also get a list of these duplicated posts?
The most reliable way to do this is to use the Data Explorer:
select PostId as [Post Link] from PostLinks where RelatedPostId=##OriginalPostId## and LinkTypeId=3 -- duplicate
This will give you both a total count for questions closed as duplicates of a given original post, and links to all of the ones that haven't been deleted yet.
There is a record on all linked questions, both linked from the given question to others and vice versa. You can see the full list by directly browsing to this URL:
All questions marked
[duplicate] are duplicates of the question having the ID in the URL.
If you're already in a question page there is "Linked" section when there are any linked posts:
And "see more linked questions…" when there are more than 10. (leading to the above page)