Pull Names from Sheet1 Name List if they don't already exist in the column MS Excel

IN MS Excel....

I want to populate G1 of Sheet3 with the value of A1 of Sheet1, if that value doesn't already exist in Column G of Sheet 3. If it DOES exist, I want to populate G1 with the value of A2 of Sheet1, if that value doesn't already exist in Column G, etc.

This is new territory for me so I don't really know where to start with this coding. I'm guessing its a lot of nested IF/THEN statements or VLOOKUP .... but I am unfamiliar. TIA!