message: Shortcircuit addTypesUsed when type has already been added
  Fixes
diff: @@ -92,6 +92,8 @@ export class Compiler {
  }
  addTypeUsed(type) {
  + if (this.typesUsedSet.has(type)) return;
  +
  if (type instanceof GraphQLEnumType ||
  type instanceof GraphQLInputObjectType ||
  (type instanceof GraphQLScalarType && !isBuiltInScalarType(type))) {
  