A (single-level) Map is generally understood to have unique keys, which allows for some invariants derived from this property, such as a one-to-one or one-to-many object relationship, without runtime checks in client code.
I do think it's worthwhile to ask the question, though. I have a strong personal preference to avoid encoding invariants like this in wire representations, because they are properties of the _schema_ and not the wire type (JSON object, in my most commonly-encountered case).
Of course, the entire point of GraphQL is to encode invariants in the data schema, so it's entirely reasonable to ask for a built-in Map type instead of building your own for every project.
JSON (as defined by json.org), for example, does not specify that key/value pairs need to be unique. ECMA-404 explicitly calls this out as an implementation consideration. I know of one implementation that supports multiple entries with the same key.
I do think it's worthwhile to ask the question, though. I have a strong personal preference to avoid encoding invariants like this in wire representations, because they are properties of the _schema_ and not the wire type (JSON object, in my most commonly-encountered case).
Of course, the entire point of GraphQL is to encode invariants in the data schema, so it's entirely reasonable to ask for a built-in Map type instead of building your own for every project.