Add suffix to column in Power Query if duplicates

Hello!

   I have data that looks like this:

I want to pivot it to look like this:

I.e. "completed date"'s become the values for the rows of protocol id's and columns of milestones.

As you can see, it pretty much works.  Problem is, every once and a while I have duplicates like this:

Where for a specific combination of protocol_id and milestone, I'll have two different dates.   I can clean up my data for now, but knowing the data source, it'll happen again.

So I was thinking one way to handle this might be to 'tag' them with indexes, maybe like this:

So that when I pivot it, I'll get a few more columns.  I may not use ALL of the columns, but at least I wont have errors.

I tried to follow this article to do so: https://excelguru.ca/label-duplicates-with-power-query/

but right when I got to the 

• Table.AddIndexColumn( [Data] , "Instance" , 1 )  step--- i started getting errors, specifically this one:
• 

   

I wonder if it has something to do with the way i grouped (by two variables).  Any ideas or suggestions?