To duplicate a lead click on the Duplicate button in the Actions widget on your lead:

A popup window appears as shown in the below screenshot.

Enter the name of the new lead, select the new lead's status and the items that will be copied to the lead, and click Duplicate:

If you choose to copy the lead's custom properties, you can either select specific properties to transfer to the new lead or copy all of them:

Note: If you're an administrator, you will also see a gear icon on the popup window which you can use to configure the default items that are copied to the lead duplicate:

When you click on the gear icon, a new popup window appears where you can select your defaults and save them: 

Once a lead is duplicated, a link to the lead is displayed next to the lead's name.

When you click the link a popup window appears showing the linked lead(s):