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The rjson::set() *sounds* like it can set any member of a JSON object (i.e., map), but that's not true :-( It calls the RapidJson function AddMember() so it can only add a member to an object which doesn't have a member with the same name (i.e., key). If it is called with a key that already has a value, the result may have two values for the same key, which is ill-formed and can cause bugs like issue #9542. So in this patch we begin by renaming rjson::set() and its variant to rjson::add() - to suggest to its user that this function only adds members, without checking if they already exist. After this rename, I was left with dozens of calls to the set() functions that need to changed to either add() - if we're sure that the object cannot already have a member with the same name - or to replace() if it might. The vast majority of the set() calls were starting with an empty item and adding members with fixed (string constant) names, so these can be trivially changed to add(). It turns out that *all* other set() calls - except the one fixed in issue #9542 - can also use add() because there are various "excuses" why we know the member names will be unique. A typical example is a map with column-name keys, where we know that the column names are unique. I added comments in front of such non-obvious uses of add() which are safe. Almost all uses of rjson except a handful are in Alternator, so I verified that all Alternator test cases continue to pass after this patch. Fixes #9583 Refs #9542 Signed-off-by: Nadav Har'El <nyh@scylladb.com> Message-Id: <20211104152540.48900-1-nyh@scylladb.com>