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You can correct the situation by filing a form W-2C (corrected W-2) and sending it in with a form W-3C. This form basically amends the W-2's that were filed earlier. You will also give a copy of the W-2C to the employees and refund to them the excess social security withholdings. The employees will not need to amend their personal tax returns because the corrected W-2 will not cause any differences on their returns.
I might also add that the 4.2% social security rate for employees has been extended for the entire year of 2012 so if you have been withholding too much so far in 2012, that excess should also be repaid to the employee.