Your edit did more than just add links - you fixed other issues with the post.
I cannot say the same of the next edit. I would love to hear why two high reputation users approved an edit that adds redundant links and nothing else. How does that improve the post? I think you did the right thing by rejecting that edit.
Now, to actually answer your question, I ...
I think there are a couple of quick points that will let us deal with these easy.
If the post already has duplicate links, don't edit them out unless other edits to improve the post are also being made.
If the post is being edited to include duplicate links and no other significant edits are being made the edit should be rejected/rolled back.
In the long ...
I don't think we need a policy on how often a link is included in a question or answer. That sounds like overregulation (which IMO is awfully common on Wikipedia).
What should be prevented is unnecessary edits, like the one you're pointing out.
If someone edited the question in its current form, just to remove the duplicate links, that would be as bad as ...