Mabey there is something about the hidden timestamp and the stored value.
Could you check the HTML source of your form and look for the hidden input field 'Timestmp' and after saving the record check what is stored in your database?
And even after that what value is in your hidden field for the second edit.
As Andy dug out there should be a comparison of the saved timestamp and the actual timestamp and it looks like there is a glitch.

Regards

Niko