Log warning if overlapping schema paths defined

Author: jiholee17

graphql-dgs-codegen-core/src/main/kotlin/com/netflix/graphql/dgs/codegen/CodeGen.kt

@@ -191,6 +191,8 @@ class CodeGen(
      * the schema information for the parser.
      */
     private fun loadSchemaReaders(vararg readerBuilders: MultiSourceReader.Builder) {
+        validateNoOverlappingPaths(config.schemaFiles)
+
         readerBuilders.forEach { rb ->
             val schemaFiles =
                 config.schemaFiles
@@ -209,6 +211,28 @@ class CodeGen(
         }
     }
 
+    /**
+     * Validates that there are no overlapping paths in the schema files to prevent duplicate loading.
+     * Throws an [IllegalArgumentException] if any path is an ancestor of another.
+     */
+    private fun validateNoOverlappingPaths(schemaFiles: Set<File>) {
+        val canonicalPaths = schemaFiles.map { it.canonicalFile }
+
+        for (i in canonicalPaths.indices) {
+            for (j in i + 1 until canonicalPaths.size) {
+                val path1 = canonicalPaths[i]
+                val path2 = canonicalPaths[j]
+
+                if (path1.startsWith(path2) || path2.startsWith(path1)) {
+                    val (parent, child) = if (path1.startsWith(path2)) path2 to path1 else path1 to path2
+                    logger.warn(
+                        "Schema path '$child' is a descendant of '$parent'. Remove one of these paths from your configuration to avoid loading duplicate schema files.",
+                    )
+                }
+            }
+        }
+    }
+
     private fun generateJava(): CodeGenResult {
         val definitions = document.definitions
         // data types

graphql-dgs-codegen-core/src/main/kotlin/com/netflix/graphql/dgs/codegen/generators/kotlin/KotlinDataTypeGenerator.kt

@@ -266,12 +266,14 @@ abstract class AbstractKotlinDataTypeGenerator(
         }
 
         val unionTypes =
-            document.getDefinitionsOfType(UnionTypeDefinition::class.java).filter { union ->
-                union.memberTypes
-                    .asSequence()
-                    .map { it as TypeName }
-                    .any { it.name == name }
-            }
+            document
+                .getDefinitionsOfType(UnionTypeDefinition::class.java)
+                .filter { union ->
+                    union.memberTypes
+                        .asSequence()
+                        .map { it as TypeName }
+                        .any { it.name == name }
+                }.distinctBy { it.name }
 
         val interfaceTypes = interfaces + unionTypes
         interfaceTypes.forEach {