How to check for duplicate rows in a column

Hi there guys, hope you are all well!!! Question--I would like to check for duplicates in a column, like the one below. I use the Excel Application Scope to create data table for it, I call it dtRequestsTemp. I already check to see if it's empty but i would like to check for duplicates as well and flag it, I don't want to delete it just flag it. But how would i go about checking for duplicate rows? Would greatly appreciate your help and thank you in advance!!
View attachment RNs.PNG
How about this:

Number position of rows
Filter collection to another coll as unique(remove duplicates)
Loop this unique collection
line by line filter collection value and get rows with same value
flag the rows where duplicates are (filtered collecion has >1 rows)