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Clone Salesforce.com Records Using a Custom Link Or Custom Button
create a custom button or custom link and then the redirect the page like:
SALESOFRCE.COM URL/RecordId/e?clone=1&retURL=RecordId
For Example:
https://ap1.salesforce.com/00590000003gTAe/e?clone=1&retURL=00590000003gTAe
if you want to clone the Account then please use the below Formula:
/{!Account.Id}/e?clone=1&retURL={!Account.Id}