When Duplicate Zapper detects a duplicate file in a Read only folder, a lock icon will be visible next to the file path. A file with a lock icon will be unavailable to check and Zap.
In order to Zap duplicate files in a Read only folder, the permissions for the folder must be changed to Read & Write. To do this, right-click on the folder and select Get Info.
Under Sharing & Permissions, change the Privilege for your OS X user account to Read & Write.
Once you have changed the Privilege to Read & Write you will be able to check the file in Duplicate Zapper for deletion and Zap the duplicate.