How do I identify duplicate values in an Excel column?

we focussed on:

only with suggestion from below

we assume that you can work on datatable base, once you had read in excel with e.g. a read range - dtData

Assign Activity:
arrDuplicatesRows | Array of DataRows - DataRow() =

(From d in dtData.AsEnumerable
Group d by k=d("ID").ToString().Trim() into grp=Group
Where grp.Count > 1
Select g = grp.Skip(1)).SelectMany(Function (x) x).toList

For each Activity: item in arrDuplicatesRows | TypeArgument: DataRow

  • Assign Activity: item(“REMARKS”) = “DUPLICATE”

As we have handled by grouping data have a look below. We can also do it within a NON-LINQ approach

when also Remark Sucess is to set, we can handle and adapt as well