Hi..

I want to have Friendly message for the below problem :

while adding documents to Document Library with same name as existing in Document Library and by unchecking "Add as new version to existing files"

FYI 

Hi,

You need to add an event receiver for this ! Please refer to the link below , it may help you :

http://zimmergren.net/technical/sp-2010-how-to-event-receivers-and-custom-error-pages

Regards,

Soumya Rao