How can I determine how a mistake occurred in my 2018 return?

[reppucci-m]

The IRS sent me notice that I owe back money from an error on my 2018 return. It seems the mortgage interest was entered as double on the 1040 but I do not know how this happened. If this was an error with the software (I filed with turbotax), what course of action can I take?

[DoninGA]

Use the TurboTax Audit Support Center and speak with an agent to get assistance with the IRS notice you received - https://support.turbotax.intuit.com/irs-notice/audit-support/

[Bsch4477]

You would have a hard time providing it was a program error since that has apparently not been widely experienced. 

[VolvoGirl]

One thing I've seen happen is if you imported any info from your bank it also imported the 1098 and Mortgage interest.  Then you also manually entered it.