Increment Number within a Concatenated String based on conditional if

I am trying to Increment a Number based on a conditional IF in another column.

In Column A I have a =Concatenate("ABC","_",D2).

I am looking to fill down a formula that will basically be an =if(C6=C5,B5,Concatenate("ABC","_",D2+1).

However, I would like to fill down across hundreds of rows. Is this possible?

Any help would be greatly appreciated. Thanks!