phpmailer的subject重复

我把phpmailer对象输出
object(PHPMailer)#8 (57) {
  ["Priority"] => int(3)
  ["CharSet"] => string(5) "utf-8"
  ["ContentType"] => string(9) "text/html"
  ["Encoding"] => string(4) "8bit"
  ["ErrorInfo"] => string(0) ""
  ["From"] => string(18) "[email protected]"
  ["FromName"] => string(6) "duncan"
  ["Sender"] => string(0) ""
  ["Subject"] => string(6) "标题"
...

["Subject"] => string(6) "标题"
但是邮件收到打开后的标题却是“标题标题”
不知为何重复了？

不清楚，可能是其它地方出错了

发邮件部分的代码贴出来看看。