Document and test first-wins lookups next to dump() and ==

dump() writes every member and == resolves duplicate keys as parse()
does, whereas lookups find the first member of a duplicate key.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_view/operator_eq.md

@@ -18,6 +18,8 @@ bool operator==(const BasicJsonType& lhs, const basic_json_view& rhs);
 Neither overload builds a `BasicJsonType` value for a view to do the comparison (see [Notes](#notes) below). Numbers
 compare by value across their types (`#!cpp 1 == 1.0`), and an object compares by its members, with duplicate keys
 resolved exactly as `parse()` resolves them -- the last value, at the position of the first occurrence of the key.
+This is not what [`operator[]`](operator[].md) and the other lookups return for a duplicate key: they find the *first*
+member (see [Duplicate keys](../../features/json_view.md#duplicate-keys)).
 
 ## Parameters