Where are your trying to unzip that zip file? on your server? using Cpanel? .
If you already have the file in your directory, then you are the better judge whether you want to really overwrite it. However, if you are unzipping in an empty directory, Then may be one should check the zip archieve.
Please brief what and where are you planning this? whether overwriting an present joomla installation or whether in to a new empty directory.
Cpanel generally doesnot allow to overwrite files from unzipped archieves. It only overwrites if you use a tar.gz or .tar formats. Thats what i have experienced.