Was this in payment of estimated income taxes, payroll taxes withheld or something else?

Use IRS form 843 Claim for refund and request for abatement to claim a refund of certain taxes, interest and penalties.

The forms lists forms 1040, 1120, 940, 941 and other original tax returns.

Instructions are listed here.

No, we had set up through turbotax an automatic payment two days before April 15th deadline and on April 15th we saw that it hadn't gone through so we paid on the IRS website. Both payment ended up going through so we double paid.

Typically, the IRS will detect a duplicate payment such as this, and send you a refund within a few weeks.

But since it's been almost a year and you haven't received the refund, you should contact the IRS.  For telephone assistance, see Telephone and Local Assistance at the IRS' Let Us Help You page.  

IRS's customer service might advise that, if you haven't filed your 2019 return yet, you might enter the overpaid amount as an estimated tax payment on your 2019 return, or as an amount applied from last year's return to your 2019 return.  But get the green light from the IRS before doing that.