Псевдокод, непроверенный: (!!!)
var myDictionary = new Dictionary<string,int>();
string col="C";
for(int row = 1; row<numberOfRows; ++row)
{
string elemValue = myWorkSheet.Range[col + row].Text.ToString();
if(myDictionary.ContainsKey(elemValue))
{
Console.WriteLine(col + myDictionary[elemValue] + " = " + col + row);
}
else
{
myDictionary[elemValue]=row;
}
}