12 lists into one while removing duplicates, how?

[Taxster]

I need to find a way to create a masterlist which is updated with only new unique entries every time I populate new lists (January - December).

I have attached an image showing what I want to do. The ID:s in the image are unique and will never have two names assigned to it.

As for name and salary columns I can link those myself, they are currently there only to illustrate the purpose.


Now to what I actually need help with:

Should I enter ID:s 1, 5, 10-18 under April (and corresponding information for name, salary etc) I want the master list to ignore ID:s 1, 5, 10-12 and only add ID:s 13-18 to the master list. How?

Thanks for any help.