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Go to Service Settings, then from the Hosted Checkout Pages section select Setup
On the Setup page, set Use Silent Post to Yes.
Check the box next to “Void transaction when my server fails to receive data sent by the silent post”. This will help prevent duplicate transactions.
Note: To ensure that transactions proceed only if your script actually receives the data returned by the Silent Post, you must Force Silent Post Confirmation by checking Void transaction when my server fails to receive data sent by the silent post.
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