How do you have multiple out comes for the same if condition in

Hello,

I have a condition in my file such that =IF(C5=C4,D3+1,D5=1 ->>>> trying to add this andD4= D3+1)

Basically this what I want to happen Column C D3

12345 1
12345 2 <-formula starts here
12345 3
12346 1
How do I have two outcomes when a condition is met?

Thanks!