How do I search through duplicate cells and alter a corresponding cell?

I've attached a picture as an example. I need to search through thousands of records and find the duplicate project managers then I need to change the project names for only duplicate project managers to say "multiple". After that I need to delete the duplicate rows based on the project manager but keep the row that has the oldest project end date. 

For example, with project manager Sam, I would keep the row with the project end date of 11/21/2018 and have the project named changed from A1 to "multiple". 

Steve and Adam wouldn't be affected as they don't have their name listed more than once. 

How can I do this?

Thank you so much for your help!