How can I find duplicate values in a column and then change the value of a cell?

Hey guys, I'm sorry for what I think is a very newbie question, but I'm not sure where else to ask, and I'm not even sure how to search for what I'm looking for. I'm looking to see if someone can tell me if this is possible, and if it is, where I should start looking to create it.

I have a call reporting system that exports call records into an excel spreadsheet. There is a timestamp column in these reports. What I'm looking to do is have some sort of macro/programmatic function loop through every row in the report. When it finds a row that has the exact same date/time stamp as the row above it (and only the row above it... though I guess this wouldn't really matter because if the row wasn't directly above it, the time/date stamp would be different), I need to change the value in a cell. To give an example:

07-06-11 12:22:38 PM | 24s call duration | type incoming
07-06-11 12:22:38 PM | 24s call duration | type outgoing

In the above scenario, I would need to change the word outgoing to the word voicemail. Is that possible to do?