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

Assisted-by: GLM 5.3

decode.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"math"
 	"reflect"
+	"slices"
 	"strings"
 	"sync"
 	"time"
@@ -323,6 +324,15 @@ func newStructSchema(t reflect.Type) structSchema {
 	return s
 }
 
+// ownsKey reports whether the field at path is the one that resolves key.
+// The encoder consults it to emit exactly the field the decoder would fill,
+// so a struct with two fields mapping to one key does not marshal into a
+// duplicate TOML key.
+func (s structSchema) ownsKey(key string, path []int) bool {
+	loc, ok := s.byName[key]
+	return ok && slices.Equal(loc.index, path)
+}
+
 // fieldByIndex walks an index path from a struct value, allocating nil
 // pointers along the way so a key can reach through an embedded pointer
 // struct. Every field on the path is exported, so each step is settable.