The most common double entry or omission error would be on the brokerage statements. If the taxpayer is automatically uploading tax forms into any tax program, it is their responsibility to double check the entries. It would be tedious doing this with dozens if not hundreds of transactions, but you can quickly review the entries by comparing the total long term and short term gains or losses on the 1099-B to what is on the tax return. You would go to the summary of your 1099-B and get the totals there.
You wont see double entries of other forms besides that, but you will have to compare the boxes in your W-2 for example, to what was uploaded. Always proofread any tax document you have entered.