Doesn't show error if submit the quote that the quote ID existed (demo site)

Hi, I am testing the function on demo site.

I create a new quote. It generate a new quote ID but, if I rewrite the quote ID which is existed in the quote list and click “save”. The page will stuck and no error being shown. For example, enter quote ID “12” which existed already in the quote list (10, 11, 12, 13) then issue will appear after clicking “save” button.

Please try to test it and fix it if can. Thanks.