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No, 6a is not doubled. The problem is the taxable amount, not the total amount. The TT worksheet is the problem as I described.

I did not use "the worksheet on the back of the SSA-1099." My SSA-1099 has no such worksheet. I used the worksheet the IRS has posted. I put a link to it in my first comment on this problem. IMO, it's not that complicated, and TT form gets it wrong it even when I don't double total amount.

Your last paragraph is exactly what IRS form does.

I have no time or interest (or skill!) to figure where the problem is in TT worksheet that it gives me a taxable amount almost 10% higher than IRS form. Here's screen shot of key rows in IRS worksheet: