This thread is resolved. Here is a description of the problem and solution.
Problem: I get 404 error in duplicated English post translation
Solution: The client had the "Redirection" plugin installed and were set some redirection rules.
I disabled those rules for the moment and then the links worked just fine, as expected. (https://wpml.org/forums/topic/404-error-in-duplicated-english-post/#post-4378715)
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