typecheck: reject duplicate class fields

README.md

@@ -181,6 +181,8 @@ The same declaration check applies to object methods in both syntax styles. For
 
 Object methods must also have unique names within their class. If `CLASS Player` declares `TEACH show` twice, `claro typecheck` reports `Method Player.show is declared more than once. Give each method a different name.` Rename one method before calling it.
 
+Class fields must also have unique names. If `CLASS Player` declares `HAS score NUMBER` twice, or declares the same field with another type, `claro typecheck` reports `Class Player declares field score more than once. Give each field a different name.` Keep one `HAS` line for each field.
+
 Simple functions and object methods can also declare a return type. `claro typecheck` checks each `RETURN` expression against it and reports a missing return when a declaration never returns a value:
 
 ```claro