Most Salesforce has more than one channel where Lead can be created. That is why it is common to have duplicate leads. You can use the “Find Duplicate” button but you will need to click on each record one by one.
Determine a common pattern to recognise duplicate lead. For example Email address. Create a trigger that does the following:
- A Lead is converted to an Account and a Contact
- The trigger can then search the Lead database for other records that share the same email address
- Convert all the duplicate leads and merge them into the Converted Account and Contact