I entered an amount of what I contributed as tax instalments at the beginning of the program and Turbotax more than doubled the amount in its calculation.

Hi,

After entering the tax instalments at the beginning, did you opt for CRA autofill? If yes, then on accepting the process, the tax instalment would have got entered again hence doubled.

You can go to line 476 of your T1 return and edit the Tax instalment amount.

Hope this helps

Thanks