Verify type definitions do not have duplicate fields. (#3925)

schema/parse.go

@@ -322,6 +322,16 @@ func parseTypeDeclaration(it *lex.ItemIterator) (*pb.TypeUpdate, error) {
 			}
 			it.Prev()
 
+			fieldSet := make(map[string]struct{})
+			for _, field := range fields {
+				if _, ok := fieldSet[field.GetPredicate()]; ok {
+					return nil, it.Item().Errorf("Duplicate fields with name: %s",
+						field.GetPredicate())
+				}
+
+				fieldSet[field.GetPredicate()] = struct{}{}
+			}
+
 			typeUpdate.Fields = fields
 			return typeUpdate, nil
 		case itemText:

schema/parse_test.go

@@ -672,6 +672,18 @@ func TestParseNonNullableScalarAndList(t *testing.T) {
 	}, result.Types[0])
 }
 
+func TestParseTypeDuplicateFields(t *testing.T) {
+	reset()
+	_, err := Parse(`
+		type Person {
+			Name: string!
+			Name: string
+		}
+	`)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "Duplicate fields with name: Name")
+}
+
 func TestParseTypeErrMissingNewLine(t *testing.T) {
 	reset()
 	_, err := Parse(`