I recently implemented section 9.5.11 of the spec inside JavaScriptCore (https://tc39.github.io/ecma262/#sec-proxy-object-internal-methods-and-internal-slots-ownpropertykeys).
Sections 17 and 19 perform the Remove operation on the List type.
I couldn't find documentation on the formal semantics of the List data structure.
Specifically, how is the Remove operation defined? Is it defined to just delete the first instance of something in the List or all instances?

Also, depending on the semantics, the algorithm defined in 9.5.11 allows for duplicate entries to be returned by this trap under certain circumstances. Is this intended? I think this would be the only place where [[OwnPropertyKeys]] has duplicate entries. Maybe we should guarantee the items are unique?