Most implementations that allow them, from those I've seen only take the last (in actual order) value...
No, it can be a gotcha because the json column type (which still exists and many may mistakenly use instead of the jsonb type) allows it:
select '{"foo": 1, "foo": 2}'::json; json ---------------------- {"foo": 1, "foo": 2} (1 row)
select '{"foo": 1, "foo": 2}'::jsonb; jsonb ------------ {"foo": 2} (1 row)
So technically, the gotcha is you have to remember to use jsonb instead of json, as well as json having a true "gotcha" and jsonb not having one.
the json type just stores the string value passed to it, but only the final value is available to the JSON-aware operators.
mydb=# select
('{"foo": 1, "foo": 2}'::jsonb)->'foo' jsonb_foo,
('{"foo": 1, "foo": 2}'::json)->'foo' json_foo;
jsonb_foo|2
json_foo|2
> 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.