I am using SF Professional edition have been Searching a free app to detect and eliminate the duplicates mainly for the Accounts, Contacts and Opportunities. Most of the free apps tends to search duplicates only during an insert or an update operation, suggest me a free app that detect the duplicates from the existing records.
With Professional Edition, you are fairly limited. Maybe you can have Salesforce enable API access temporarily and utilize something like DemandTools to detect duplicate in mass? DemandTools is primarily used for mass duplicate detection and cleanup, when other tools like RingLead are used primarily for UI prevention.
perhaps this will help: Salesforce Labs Dupe Eliminator
I have a similar issue (Im sure all orgs have some duplicates) where I use a passive application like DupeCatcher. This handles de-dupeing on insert and update.
I then created a checkbox field (isDuped) on the SObject in question (account) initially set to false. Using http://Dataloader.io I exported the accounts 10k at a time and upserted with the checkbox isDuped set to true. This then triggered the ded-duplication to run on each account that was upserted.
A poor mans solution :)