Try this article. It's best practice to merge duplicates, not delete them. Also, if you use SalesForce you should be fine but we use another CRM system which hooks up the lead ID - we can't delete or merge our leads as it causes our CRM to fail when sending campaigns as it can't find the lead ID anymore. Just check you won't have any problems before you start 🙂
https://community.marketo.com/MarketoArticle?id=kA050000000Kyu3CAC