fix: deduplicate interfaces implementing interfaces (#608)

src/builder.ts

@@ -1055,7 +1055,7 @@ export class SchemaBuilder {
       const type = this.getInterface(i)
       list.push(type, ...type.getInterfaces())
     })
-    return list
+    return Array.from(new Set(list))
   }
 
   protected buildInterfaceFields(interfaces: (string | NexusInterfaceTypeDef<any>)[]) {

tests/__snapshots__/interfaceType.spec.ts.snap

@@ -26,6 +26,31 @@ Object {
 
 exports[`interfaceType can not implement itself 1`] = `"GraphQL Nexus: Interface Node can't implement itself"`;
 
+exports[`interfaceType deduplicates interfaces implementing interfaces 1`] = `
+"### This file was generated by Nexus Schema
+### Do not make changes to this file directly
+
+
+interface Node {
+  id: ID
+}
+
+interface Node2 implements Node {
+  id: ID
+  id2: ID
+}
+
+type Foo implements Node2 & Node {
+  id: ID
+  id2: ID
+}
+
+type Query {
+  ok: Boolean!
+}
+"
+`;
+
 exports[`interfaceType detects circular dependencies 1`] = `"GraphQL Nexus: Interface circular dependency detected NodeA -> NodeC -> NodeB -> NodeA"`;
 
 exports[`interfaceType logs error when resolveType is not provided for an interface 1`] = `

tests/interfaceType.spec.ts

@@ -1,6 +1,6 @@
 import { graphql } from 'graphql'
 import path from 'path'
-import { interfaceType, makeSchema, objectType, queryField } from '../src/core'
+import { interfaceType, makeSchema, objectType, queryField, generateSchema } from '../src/core'
 
 describe('interfaceType', () => {
   it('can be implemented by object types', async () => {
@@ -127,6 +127,38 @@ describe('interfaceType', () => {
       })
     ).toThrowErrorMatchingSnapshot()
   })
+  it('deduplicates interfaces implementing interfaces', async () => {
+    const { schemaTypes } = await generateSchema.withArtifacts(
+      {
+        types: [
+          interfaceType({
+            name: 'Node',
+            definition(t) {
+              t.id('id')
+              t.resolveType(() => null)
+            },
+          }),
+          interfaceType({
+            name: 'Node2',
+            definition(t) {
+              t.implements('Node')
+              t.id('id2')
+              t.resolveType(() => null)
+            },
+          }),
+          objectType({
+            name: 'Foo',
+            definition(t) {
+              t.implements('Node2', 'Node')
+            },
+          }),
+        ],
+        outputs: false,
+      },
+      false
+    )
+    expect(schemaTypes).toMatchSnapshot()
+  })
   it('detects circular dependencies', async () => {
     expect(() =>
       makeSchema({