Calculating total overtime hours worked per employee

I need to calculate the total number of overtime hours worked in a 4 week period using employee numbers to identify the worker. Each employee may have multiply entries.

I have absolutely no idea how to get the calcuation to look through the spreadsheet looking for duplicate employee numbers and totalling their hours. (Full spreadsheet can have over 1000 entries)

Any help would be much appreciated. I have attached a sample spreadsheet.

Thanks