How do I respond to an IRS letter asking me to submit additional information to substantiate a claim for excess social security and tier 1 RRTA tax withheld?

[xmasbaby0]

What tax year?  

Check the worksheets from your tax return to see if you entered all of the W-2 income for both spouses under only one of the names on a joint return.  That is the most common reason for this problem.  Unfortunately, it is user error.  As you prepare your return, several screens alert you to it. 

Look at your actual W-2's--check box 4 on each one to see how much SS was withheld.  If the sum of the amounts in box 4 for ONE of you is over the maximum for the tax year, then you really had excess tax withheld for that one person:

2016  Maximum $7347      2017 Maximum $7886         2018    $7960.80 

BUT if you entered all of the W-2's on a joint return under only one spouse's name, it inflated your refund, and now the IRS wants that money back.

[xmasbaby0]

If in fact you did make the mistake of putting all of the income under only one name instead of under each spouse's name, and you made that mistake on an earlier year, better check the subsequent years to see if you did it for those returns, too, so that you can amend, pay,  and avoid penalties before the IRS catches the mistake.