Formula to calculate the count of repeating rows

Appreciate if anyone can help me with below. Thanks

I have two columns that i need for an upload. Column C1 contains ID's in numeric and column C2 is incremented everytime when there is a duplicate C1. The increment breaks and starts fresh when there is a new value in C1.

The output needs to be as below:

C1 C2
1 1
1 2
1 3
1 4
1 5
1 6
2 1
2 2
2 3
2 4
2 5
2 6
2 7
3 1
3 2
3 3
3 4
3 5
3 6
4 1
4 2
4 3
4 4
4 5
4 6

Thanks it worked.