diff --git a/validator/schema.go b/validator/schema.go
index 1780ae36..33bf41e2 100644
--- a/validator/schema.go
+++ b/validator/schema.go
@@ -199,6 +199,14 @@ func validateDefinition(schema *Schema, def *Definition) *gqlerror.Error {
 		}
 	}
 
+	for idx, field1 := range def.Fields {
+		for _, field2 := range def.Fields[idx+1:] {
+			if field1.Name == field2.Name {
+				return gqlerror.ErrorPosf(field2.Position, "Field %s.%s can only be defined once.", def.Name, field2.Name)
+			}
+		}
+	}
+
 	return validateDirectives(schema, def.Directives, nil)
 }
 
diff --git a/validator/schema_test.yml b/validator/schema_test.yml
index 375fb640..dba1040d 100644
--- a/validator/schema_test.yml
+++ b/validator/schema_test.yml
@@ -10,6 +10,38 @@ types:
     error:
       message: "Cannot redeclare type A."
       locations: [{line: 4, column: 6}]
+  - name: cannot be duplicated field at same definition 1
+    input: |
+      type A {
+        name: String
+        name: String
+      }
+    error:
+      message: "Field A.name can only be defined once."
+      locations: [{line: 3, column: 3}]
+  - name: cannot be duplicated field at same definition 2
+    input: |
+      type A {
+        name: String
+      }
+      extend type A {
+        name: String
+      }
+    error:
+      message: "Field A.name can only be defined once."
+      locations: [{line: 5, column: 3}]
+  - name: cannot be duplicated field at same definition 3
+    input: |
+      type A {
+        name: String
+      }
+      extend type A {
+        age: Int
+        age: Int
+      }
+    error:
+      message: "Field A.age can only be defined once."
+      locations: [{line: 6, column: 3}]
 
 object types:
   - name: must define one or more fields