Excessive questions about "file changed on disk" in tramp ftp

[XeCycle]

Hello,

I'm using TRAMP to edit files on a ftp server.  On saving the
file Emacs usually asks me "file changed on disk.  really save?"
But I'm the only one editing that file, of which I can be sure.
By "usually" I mean almost every time.

Something must be wrong here.  Where shall I start with
debugging?

Thank you.

-- 
Carl Lei (XeCycle)
Department of Physics, Shanghai Jiao Tong University
OpenPGP public key: 7795E591
Fingerprint: 1FB6 7F1F D45D F681 C845 27F7 8D71 8EC4 7795 E591