I've spent too much time going around in circles on this - so time to engage the hive mind :)
Simple to explain (I hope), but hard for me to find a way to get the effect I need :(
"Identity the latest entry for [Initials] if previous entries for [Initials] cover the same date range"
So [Row_ID] 15 is the latest entry for [Initials]=RS and [Row_ID]'s 17 and 18 are the latest entries for [Initials]=AA
Need a column formula that will allow me to filter/move rows out of the sheet so that I'm left with (in the example above) just [ROW_ID]'s 15,17 and 18