Check Your Duplicates Against Your Statement
Review your paper or online bank statement at the end of each month and match up your duplicate checks to the amounts listed on the statement. If everything matches up, you can discard the duplicate checks, unless you use that account for your business or to pay for tax-deductible expenses.
Keep Duplicates for Deductible Expenses Longer
If you paid for eligible business-related goods or services with a check and plan to make deductions for those expenses, you should save your duplicate checks for far longer, as the IRS considers those duplicate checks "proof of payment" for those deductible purchases. Save business expense records for a minimum of six years or, even better, indefinitely.
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Bankrate recommends retaining all duplicate checks for deductible expenses such as charitable donations, business expenses, alimony and mortgage payments for at least six years, if not indefinitely.