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If the transaction in your account history has a status of "completed", your only option is contact the receiver and request that they send the money back. If the status shows pending or unclaimed and the "cancel" link is active, you can cancel the transaction. If you cancel it and the status remains unclaimed, PayPal will cancel it and return the funds.
Slight correction cause I can't type and think at the same time:
If you cannot cancel it and the status remains unclaimed for 30 days, PayPal will cancel it and return the funds.