If an endoscopy is already negative, why is it necessary to perform a fluoroscopy of upper gi with kub. patient basically has cough with blood post extubation. ct of chest and throat came negative. is it like overdoing?

If by Endoscopy you mean an Esophagogastroduodenoscopy has been performed, then "Yes" an UGI study would be redundant in this case. However, if by Endoscopy you mean something like a Laryngoscopy or a Bronchoscopy was done, then an UGI study is a reasonable test to perform, which may even then be followed by an EGD. Can you clarify which particular Endoscopic procedure has been done already?

Dr Marra and another doctor agreed

I do agree with Dr. Stalin's answer that if endoscopy looked at esophagus, stomach and duodenum, that is usually sufficient to detect blood from upper GI tract. It does sound like the combination of cough and blood, raised the question "Is the blood coming from lungs through the trachea, or is it instead coming from the GI tract?". Sometimes fluoro imaging offers a useful alternate perspective.