How can I check a duplicate file or image being uploaded into a Plone site
You can store hash ( eg md5 ) of your image content into a database, at upload time.
When a new image is uploaded, you just have to match the current image's hash against those present in the database.
The hash will be collision free mostly, however to be more sure, you can also store other attributes like filesize, image dimensions in the same database and compare them as a double check.
How can I check a duplicate file or image being uploaded into a Plone site. I am using Plone 4.1? Is there some feature  or configuration changes that need to be done to achieve the same.I want this feature to check the entire site before uploading any file /image into the site. I am working on Linux OS. I am using the Zope server which gets installed with Plone 4.1.4.