Assign number to row based off two unique columns (i.e. 0-10)

I am trying to reassign a column that contains an index, or create new and replace the existing column, that is based off ID and ID2.  There is probably something very easy I am missing as I am fairly new to Alteryx. 

  ID          ID2        Index                              

 660862    0          10             - the 10 would become 0
 660862    0          11 
 660862    0          12 
 660862    0          13 
 660862    0          14 
 660862    0          15 
 660862    0          16             - result would be 6; end of first unique

 660862    1          21             - new unique pair the 21 would become 0 and count up until next unique pair

Any input would be greatly appreciated!

Thanks,

Aaron