Does my mom need to pay taxes on the sale of her duplex? It is her primary residence, but she rents out the other side. She does claim expenses for the rental on taxes.

Technically the one sale is reported in 2 parts on the tax return ... the rental sale and the personal sale.  This can be confusing so I highly recommend she seeks local professional assistance  or  upgrades to the LIVE option for one on one assistance. 

The bottom line answer to your question is yes, the sale must be reported no matter what. Exactly how it's reported depends on how the income from one side of the duplex was reported and how things were set up on the SCH E. So for this year you might find it cheaper in the long run to seek professional help with taxes this year. Especially if your mom's state also taxes personal income.

she should have been claiming depreciation on the rental portion.   even if she didn't, if she has a gain on that portion,  the depreciation she should have taken but didn't, is subject to recapture.  if she failed to properly report the rental she should consult a tax pro to help her out of potentially serious tax issues. 

Thanks for all the responses.  I was able to talk to my mom's accountant about this and the answer is Yes.  She does have to pay capital gains taxes to both the Feds and the state of IL on ½ of the sale price of the duplex due to it being a rental.  The math is more complicated than that as the taxable amount is modified by the original price of the home, the value it held when my dad died back in 2008 and the final sale price minus all commission, closing costs, etc... from the sale.