RFC4627 §2.2 ¶1

> The names within an object SHOULD be unique.

RFC2119 §3

> [SHOULD], 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.

Not allowing duplicate keys in JSON objects is very close the exact opposite of a gotcha. Allowing and round-tripping them would be a gotcha.