Billed twice for one visit - how do I dispute this?

Hi, so today my psychiatrist's office suddenly messaged me saying that I owed a few hundred dollars and because I was delinquent on these charges they could not continue to provide services/prescribe until I paid them. This surprised me, as before every visit (which is virtual) I have paid the co-pay and have never had any issues. Upon inquiring further, I found out that because of the way they billed each visit I was supposed to pay two co-pays for one single virtual visit. One billing code that was used was for the psychiatric appointment, and then the other was for an outpatient visit because it's being done virtually. The billing department at said office failed to bill me for one billing code until now, which now makes it look as though I have not been paying for these appointments for months (that I don't even think is a service that should get billed separately at this point). I would love to dispute this, but I have no idea how and it seems pretty strange because when I had a prior psychiatrist I only was billed once even for virtual visits. If anyone knows of any way to fight this (or if this is just going to be something I will have to deal with), please let me know 🙁

In the meantime I am going to be moving soon anyway so I will be finding another doctor, but I had figured I'd try to continue to see this doctor until I moved.

Thanks in advance!

Edit: Also, to add to this, because they forgot to bill this, none of this information is on my EOB. I expect it to be put on my EOB for this upcoming bill…