Saving product display error "The content on this page has either been modified by another user"

Hi,

This actually happen to me few times. Whenever I'm adding a product display and a variation, if i stay on that page for a longer period, error will come out when I clicked "Save".

Error message: The content on this page has either been modified by another user, or you have already submitted modifications using this form. As a result, your changes cannot be saved.

and the variation will create a duplicate with incomplete information. look at the screenshot below.

I'm VERY sure i'm the only one who creating product display. Don't know why this happen.

Any help will be greatly appreciate.

Thanks.
Max.