Remove false positive in circular structure detection (#16380)

Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

packages/next/lib/is-serializable-props.ts

@@ -98,11 +98,16 @@ export function isSerializableProps(
     if (Array.isArray(value)) {
       visit(refs, value, path)
 
+      const seen = new Set<any>()
       const newRefs = new Map(refs)
       if (
-        value.every((nestedValue, index) =>
-          isSerializable(newRefs, nestedValue, `${path}[${index}]`)
-        )
+        value.every((nestedValue, index) => {
+          if (seen.has(nestedValue)) {
+            return true
+          }
+          seen.add(nestedValue)
+          return isSerializable(newRefs, nestedValue, `${path}[${index}]`)
+        })
       ) {
         return true
       }

test/unit/is-serializable-props.test.js

@@ -275,4 +275,13 @@ Reason: Circular references cannot be expressed in JSON (references: \`.k\`)."
       })
     ).toBe(true)
   })
+
+  it('allows identical object instances in an array', () => {
+    const obj = { foo: 'bar' }
+    const arr = [obj, obj]
+    const objWithArr = { deep: { arr } }
+
+    expect(isSerializableProps('/', 'test', { arr })).toBe(true)
+    expect(isSerializableProps('/', 'test', { objWithArr })).toBe(true)
+  })
 })