Improve Schema Transformation

Do not pass the query, mutation and subscription also in additionalTypes
when constructing the schema. This avoids problems with duplicate types
when transforming the schema.

Fixes #300

Author: mkaulig

src/main/kotlin/com/coxautodev/graphql/tools/SchemaParser.kt

@@ -119,14 +119,16 @@ class SchemaParser internal constructor(scanResult: ScannedSchemaObjects, privat
         val mutationName = rootInfo.getMutationName()
         val subscriptionName = rootInfo.getSubscriptionName()
 
-        val query = objects.find { it.name == queryName }
+        val query : GraphQLObjectType= objects.find { it.name == queryName }
                 ?: throw SchemaError("Expected a Query object with name '$queryName' but found none!")
         val mutation = objects.find { it.name == mutationName }
                 ?: if (rootInfo.isMutationRequired()) throw SchemaError("Expected a Mutation object with name '$mutationName' but found none!") else null
         val subscription = objects.find { it.name == subscriptionName }
                 ?: if (rootInfo.isSubscriptionRequired()) throw SchemaError("Expected a Subscription object with name '$subscriptionName' but found none!") else null
 
-        return SchemaObjects(query, mutation, subscription, (objects + inputObjects + enums + interfaces + unions).toSet(), codeRegistryBuilder)
+        val additionalObjects = objects.filter { o -> o != query || o != subscription || o != mutation }
+
+        return SchemaObjects(query, mutation, subscription, (additionalObjects + inputObjects + enums + interfaces + unions).toSet(), codeRegistryBuilder)
     }
 
     /**