IMHO, the complexity that this would introduce just highlights why it's a good idea not to have them.

> "but I think it would be legal to reject JSON with duplicate keys."

No it would not. `SHOULD` has a very specific meaning:

  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.

See http://www.ietf.org/rfc/rfc2119.txt

An object with duplicate keys must be parsed, but the exactly result will be implementation specific (undefined).