Made a duplicated payment - how to get my money back?

Hi all,

 

I received a CP-2000 notice for an old tax return - I sent a check... which was not getting processed, so I made an online payment, thinking that the check was lost.

2 months later, I noticed that they actually just cleared my check. As a consequence, I paid twice the amount.

 

Do you know the process to get my money back?

 

thank you!