Excel Helper

I have a list of names in column A.

What i want is program whereby if I enter a new name at the bottom of the
column, if it is a duplicate (i.e. name appears elsehwere in list) then a
msgbox appears telling em it is a duplicate.

Now I can actauly ahcieve this in VBA but...

I have to actaully execute the macro maually.

How can i get the macro to run automatically i.e. I enter a new name, press
enter, and the macro runs without me clicking a separate button to run it of
pressing F5 etc.

I have tried...

Application.Volatile
Application. OnKey
Application. Onaction

How do I get Excel to run a macro once an event has occurred.

Any insights appreciated...

Alex