OK ... first if they filed a second 2019 return using the 2018 information it will be ignored since you cannot file the same tax year twice unless you amend the original return. If they sent a payment with the second filing then it should be returned due to the rejected return.
Next ... if they still need to file a 2018 return and they owe they need to file it ASAP even if they don't send the payment with the return ... the IRS will bill them later. However any refund is forfeited at this late date.
