Go to the details of the sent campaign. In campaign statistics, click on the button More Details:

Copy the campaign via the corresponding action. Find the copied campaign under campaigns > drafts.
If you want to copy the sent email into an existing campaign draft, copy the email via the button .

Use the same target group again

With Create From Campaigns target groups can be selected from sent campaigns. Go to Target Groups > Create From Campaigns. You can choose the campaign and the email and decide between recipients, openers, clickers etc. Keep in mind that unsubscribes and hardbounces are automatically deleted from the target group.