Add Decoder.DisallowDuplicateFields to reject duplicate object keys

decode.go

@@ -252,6 +252,14 @@ func (d *Decoder) DisallowUnknownFields() {
 	d.s.DisallowUnknownFields = true
 }
 
+// DisallowDuplicateFields causes the Decoder to return an error when the
+// destination is a struct and the input object contains more than one key that
+// maps to the same field. By default the last occurrence wins, matching
+// encoding/json.
+func (d *Decoder) DisallowDuplicateFields() {
+	d.s.DisallowDuplicateFields = true
+}
+
 func (d *Decoder) InputOffset() int64 {
 	return d.s.TotalOffset()
 }

decode_test.go

@@ -289,6 +289,61 @@ func Test_Decoder_DisallowUnknownFields(t *testing.T) {
 	}
 }
 
+func Test_Decoder_DisallowDuplicateFields(t *testing.T) {
+	type nested struct {
+		B int `json:"b"`
+	}
+	type payload struct {
+		A int    `json:"a"`
+		N nested `json:"n"`
+	}
+
+	t.Run("duplicate reports an error", func(t *testing.T) {
+		dec := json.NewDecoder(strings.NewReader(`{"a": 1, "a": 2}`))
+		dec.DisallowDuplicateFields()
+		var v payload
+		err := dec.Decode(&v)
+		if err == nil {
+			t.Fatal("expected duplicate field error")
+		}
+		if err.Error() != `json: duplicate field "a"` {
+			t.Fatalf("unexpected error: %v", err)
+		}
+	})
+
+	t.Run("duplicate in a nested object is caught", func(t *testing.T) {
+		dec := json.NewDecoder(strings.NewReader(`{"n": {"b": 1, "b": 2}}`))
+		dec.DisallowDuplicateFields()
+		var v payload
+		if err := dec.Decode(&v); err == nil {
+			t.Fatal("expected duplicate field error")
+		}
+	})
+
+	t.Run("distinct fields decode normally", func(t *testing.T) {
+		dec := json.NewDecoder(strings.NewReader(`{"a": 1, "n": {"b": 2}}`))
+		dec.DisallowDuplicateFields()
+		var v payload
+		if err := dec.Decode(&v); err != nil {
+			t.Fatal(err)
+		}
+		if v.A != 1 || v.N.B != 2 {
+			t.Fatalf("unexpected value: %+v", v)
+		}
+	})
+
+	t.Run("default keeps last-wins behavior", func(t *testing.T) {
+		dec := json.NewDecoder(strings.NewReader(`{"a": 1, "a": 2}`))
+		var v payload
+		if err := dec.Decode(&v); err != nil {
+			t.Fatal(err)
+		}
+		if v.A != 2 {
+			t.Fatalf("expected last value to win, got %d", v.A)
+		}
+	})
+}
+
 func Test_Decoder_EmptyObjectWithSpace(t *testing.T) {
 	dec := json.NewDecoder(strings.NewReader(`{"obj":{ }}`))
 	var v struct {

internal/decoder/stream.go

@@ -15,17 +15,18 @@ const (
 )
 
 type Stream struct {
-	buf                   []byte
-	bufSize               int64
-	length                int64
-	r                     io.Reader
-	offset                int64
-	cursor                int64
-	filledBuffer          bool
-	allRead               bool
-	UseNumber             bool
-	DisallowUnknownFields bool
-	Option                *Option
+	buf                     []byte
+	bufSize                 int64
+	length                  int64
+	r                       io.Reader
+	offset                  int64
+	cursor                  int64
+	filledBuffer            bool
+	allRead                 bool
+	UseNumber               bool
+	DisallowUnknownFields   bool
+	DisallowDuplicateFields bool
+	Option                  *Option
 }
 
 func NewStream(r io.Reader) *Stream {

internal/decoder/struct.go

@@ -688,7 +688,7 @@ func (d *structDecoder) DecodeStream(s *Stream, depth int64, p unsafe.Pointer) e
 		seenFieldNum int
 	)
 	firstWin := (s.Option.Flags & FirstWinOption) != 0
-	if firstWin {
+	if firstWin || s.DisallowDuplicateFields {
 		seenFields = make(map[int]struct{}, d.fieldUniqueNameNum)
 	}
 	for {
@@ -720,6 +720,14 @@ func (d *structDecoder) DecodeStream(s *Stream, depth int64, p unsafe.Pointer) e
 					}
 					seenFields[field.fieldIdx] = struct{}{}
 				}
+			} else if s.DisallowDuplicateFields {
+				if _, exists := seenFields[field.fieldIdx]; exists {
+					return fmt.Errorf("json: duplicate field %q", key)
+				}
+				if err := field.dec.DecodeStream(s, depth, unsafe.Pointer(uintptr(p)+field.offset)); err != nil {
+					return err
+				}
+				seenFields[field.fieldIdx] = struct{}{}
 			} else {
 				if err := field.dec.DecodeStream(s, depth, unsafe.Pointer(uintptr(p)+field.offset)); err != nil {
 					return err