Disallow repeat of non repeatable directives

internal/schema/schema.go

@@ -289,6 +289,7 @@ func resolveField(s *types.Schema, f *types.FieldDefinition) error {
 }
 
 func resolveDirectives(s *types.Schema, directives types.DirectiveList, loc string) error {
+	alreadySeenNonRepeatable := make(map[string]struct{})
 	for _, d := range directives {
 		dirName := d.Name.Name
 		dd, ok := s.Directives[dirName]
@@ -315,6 +316,14 @@ func resolveDirectives(s *types.Schema, directives types.DirectiveList, loc stri
 				d.Arguments = append(d.Arguments, &types.Argument{Name: arg.Name, Value: arg.Default})
 			}
 		}
+
+		if dd.Repeatable {
+			continue
+		}
+		if _, seen := alreadySeenNonRepeatable[dirName]; seen {
+			return errors.Errorf(`non repeatable directive %q can not be repeated. Consider adding "repeatable".`, dirName)
+		}
+		alreadySeenNonRepeatable[dirName] = struct{}{}
 	}
 	return nil
 }

internal/schema/schema_test.go

@@ -920,6 +920,22 @@ Second line of the description.
 				return nil
 			},
 		},
+		{
+			name: "Disallow repeat of a directive if it is not `repeatable`",
+			sdl: `
+			directive @nonrepeatabledirective on FIELD_DEFINITION
+			type Foo {
+				bar: String @nonrepeatabledirective @nonrepeatabledirective
+			}
+			`,
+			validateError: func(err error) error {
+				prefix := `graphql: non repeatable directive "nonrepeatabledirective" can not be repeated. Consider adding "repeatable"`
+				if err == nil || !strings.HasPrefix(err.Error(), prefix) {
+					return fmt.Errorf("expected error starting with %q, but got %q", prefix, err)
+				}
+				return nil
+			},
+		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			s, err := schema.ParseSchema(test.sdl, test.useStringDescriptions)