diff --git a/src/diagnostic/hook-diagnostic.ts b/src/diagnostic/hook-diagnostic.ts
index a1981a9..27dc078 100644
--- a/src/diagnostic/hook-diagnostic.ts
+++ b/src/diagnostic/hook-diagnostic.ts
@@ -5,6 +5,9 @@ import { Position, Range } from '../diff'
 import { GraphQLContext, createGraphQLContext, getGQLNodeLocationRange } from '../gql'
 import { getGQLContent, getGraphQLQueryVariable, getTSNodeLocationRange } from '../ts'
 import { Diagnostic, DiagnosticSeverity } from './diagnostic-type'
+import { NoDuplicateFieldName } from './rules/NoDuplicateFieldName'
+
+const additionalRules = [NoDuplicateFieldName]
 
 function toError(error: gql.GraphQLError, context: Pick<GraphQLContext, 'offset' | 'sourceFile'>) {
   const lineOffset = context?.offset?.line ?? 0
@@ -46,7 +49,9 @@ export function diagnoseGraphQLQuery(
 ): Diagnostic[] {
   try {
     const document = gql.parse(query)
-    const result = gql.validate(schema, document)
+    const result = gql
+      .validate(schema, document)
+      .concat(additionalRules.flatMap(rule => rule(document, schema)))
     return result.flatMap(error => toError(error, context))
   } catch (e) {
     return [toError(e, context)].flat()
diff --git a/src/diagnostic/rules/NoDuplicateFieldName.ts b/src/diagnostic/rules/NoDuplicateFieldName.ts
new file mode 100644
index 0000000..8c568cc
--- /dev/null
+++ b/src/diagnostic/rules/NoDuplicateFieldName.ts
@@ -0,0 +1,36 @@
+import * as gql from 'graphql'
+import { isFieldNode, isInlineFragmentNode } from '../../gql'
+
+export function NoDuplicateFieldName(document: gql.DocumentNode, schema: gql.GraphQLSchema) {
+  const errors: gql.GraphQLError[] = []
+  const typeInfo = new gql.TypeInfo(schema)
+  gql.visit(
+    document,
+    gql.visitWithTypeInfo(typeInfo, {
+      SelectionSet(node) {
+        const parent = typeInfo.getParentType()
+        const type = parent?.name
+        node.selections.reduce(
+          (a, node) => {
+            const name = isFieldNode(node)
+              ? node.name.value
+              : isInlineFragmentNode(node)
+                ? node.typeCondition?.name.value
+                : undefined
+            if (!name) return a
+            if (!!a[name]) {
+              errors.push(
+                new gql.GraphQLError(`Duplicate field "${name}" on type "${type}" `, {
+                  nodes: [node],
+                }),
+              )
+            }
+            return { ...a, [name]: true }
+          },
+          {} as Record<string, boolean>,
+        )
+      },
+    }),
+  )
+  return errors
+}
diff --git a/src/diagnostic/rules/index.ts b/src/diagnostic/rules/index.ts
new file mode 100644
index 0000000..f21684d
--- /dev/null
+++ b/src/diagnostic/rules/index.ts
@@ -0,0 +1 @@
+export * from './NoDuplicateFieldName'