SHOULD is defined in RFC 2119 as

    3. SHOULD   This word, or the adjective "RECOMMENDED", mean that there
       may exist valid reasons in particular circumstances to ignore a
       particular item, but the full implications must be understood and
       carefully weighed before choosing a different course.

The consequences are undefined, I feel, for a reason. You can't put them all down on paper, it depends on what all the parsers do. The parsers can accept or reject things with duplicate keys, or they can play a nice little ditty through the speakers.

All it means is a parser isn't required to reject JSON with multiple keys. It can, however, do whatever the fuck it wants with them.

If the wording was precise, then it should be a MUST. SHOULD indicates a terrible world of unknown consequences.