Create a flag to keep only one row of multi entered field based on max number

Hi all,

I have a file in the form :
What I want is to keep the unique ID rows with teh maximum length of service. I've tried the MAX IF but a) it doesn't work and b it doesn't really help as I have to keep the unique IDs and the populate the maximum number, but I want to keep loads of columns next to them. So, what would ideally work is to somehow flag what to drop what to keep from the existing data set.

For example, can I add a column D with that flag which would indicate to keep rows 4,6,7 since for those ids these rows have the maximun length of service number. Then I could filter in only the "keep" flag.

Any ideas?

I don't know why I can't delete the first attachment. The second is the right screenshot to look at.