fix(encode): emit one key when two fields resolve to one name

Assisted-by: GLM 5.3

encode_test.go

@@ -383,6 +383,42 @@ func TestMarshalerErrorPropagates(t *testing.T) {
 	}
 }
 
+// Two fields that resolve to one TOML key must marshal as one key, resolved
+// the way the decoder resolves it, or the output would carry a duplicate key
+// and never re-parse.
+func TestMarshalDuplicateKeyResolvesToOneField(t *testing.T) {
+	type SameLevel struct {
+		First  int    `toml:"v"`
+		Second string `toml:"v"`
+	}
+	out, err := Marshal(SameLevel{First: 1, Second: "s"})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	if want := "v = \"s\"\n"; string(out) != want {
+		t.Errorf("output mismatch:\ngot:  %q\nwant: %q", out, want)
+	}
+
+	type Base struct {
+		Name string `toml:"name"`
+	}
+	type Embedded struct {
+		Base
+		Name string `toml:"name"`
+	}
+	out, err = Marshal(Embedded{Base: Base{Name: "inner"}, Name: "outer"})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	// The shallower field wins, matching the decoder.
+	if want := "name = \"outer\"\n"; string(out) != want {
+		t.Errorf("output mismatch:\ngot:  %q\nwant: %q", out, want)
+	}
+	if _, err := Parse(out); err != nil {
+		t.Errorf("re-parse: %v\ndoc:\n%s", err, out)
+	}
+}
+
 func TestMarshalEmbeddedScalarStruct(t *testing.T) {
 	// A field declared directly as a scalar-struct type (here LocalDateTime)
 	// must be encoded as a TOML scalar at the parent level, not rendered as