BUGFIX: Throw on duplicate derived model names

src/Schema/Schema.php

@@ -860,6 +860,16 @@ public function removeScalar(string $name): self
     public function addModel(ModelType $modelType, ?callable $callback = null): Schema
     {
         $existing = $this->models[$modelType->getName()] ?? null;
+        if ($existing) {
+            Schema::invariant(
+                $existing->getModel()->getSourceClass() === $modelType->getModel()->getSourceClass(),
+                'Name collision for model "%s". It is used for both %s and %s. Use the type_mapping setting in your schema
+            config to provide a custom name for the model.',
+                $modelType->getName(),
+                $existing->getModel()->getSourceClass(),
+                $modelType->getModel()->getSourceClass()
+            );
+        }
         $typeObj = $existing
             ? $existing->mergeWith($modelType)
             : $modelType;

tests/Fake/SubFake/FakePage.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace SilverStripe\GraphQL\Tests\Fake\SubFake;
+
+use SilverStripe\GraphQL\Tests\Fake\FakeSiteTree;
+
+class FakePage extends FakeSiteTree
+{
+    private static $table_name = "GraphQL_SubFakePage";
+
+    private static $db = [
+        'SubFakePageField' => 'Varchar',
+    ];
+}

tests/Schema/SchemaTest.php

@@ -4,12 +4,10 @@
 
 use SilverStripe\Core\Config\Config;
 use SilverStripe\Dev\SapphireTest;
-use SilverStripe\GraphQL\Schema\DataObject\DataObjectModel;
 use SilverStripe\GraphQL\Schema\DataObject\ModelCreator;
 use SilverStripe\GraphQL\Schema\Exception\SchemaBuilderException;
 use SilverStripe\GraphQL\Schema\Field\Mutation;
 use SilverStripe\GraphQL\Schema\Field\Query;
-use SilverStripe\GraphQL\Schema\Interfaces\SchemaStorageInterface;
 use SilverStripe\GraphQL\Schema\Schema;
 use SilverStripe\GraphQL\Schema\SchemaConfig;
 use SilverStripe\GraphQL\Schema\Type\Enum;
@@ -19,7 +17,9 @@
 use SilverStripe\GraphQL\Schema\Type\Type;
 use SilverStripe\GraphQL\Schema\Type\UnionType;
 use SilverStripe\GraphQL\Tests\Fake\DataObjectFake;
+use SilverStripe\GraphQL\Tests\Fake\FakePage;
 use SilverStripe\GraphQL\Tests\Fake\FakeSiteTree;
+use SilverStripe\GraphQL\Tests\Fake\SubFake\FakePage as SubFakePage;
 
 class SchemaTest extends SapphireTest
 {
@@ -259,6 +259,24 @@ public function testValidConfig()
         Schema::assertValidConfig([]);
     }
 
+    public function testDuplicateModels()
+    {
+        $schema = $this->buildSchema();
+        $schema->addModelbyClassName(FakePage::class);
+        $this->expectException(SchemaBuilderException::class);
+        $schema->addModelbyClassName(SubFakePage::class);
+
+        $schema = $this->buildSchema();
+        $schema->getConfig()->getModelConfiguration()->set('type_mapping', [
+            SubFakePage::class => 'SubFakePage',
+        ]);
+        $schema->addModelbyClassName(FakePage::class);
+        $schema->addModelbyClassName(SubFakePage::class);
+
+        $this->assertInstanceOf(ModelType::class, $schema->getModel('FakePage'));
+        $this->assertInstanceOf(ModelType::class, $schema->getModel('SubFakePage'));
+    }
+
     public static function noop()
     {
     }