stream/mts/meta.go: avoiding adding same key to order slice

stream/mts/meta/meta.go

@@ -80,6 +80,12 @@ func New() *Metadata {
 func (m *Metadata) Add(key, val string) {
 	m.mu.Lock()
 	m.data[key] = val
+	for _, k := range m.order {
+		if k == key {
+			m.mu.Unlock()
+			return
+		}
+	}
 	m.order = append(m.order, key)
 	m.mu.Unlock()
 }