Preventing duplicate entries using a data validation list

Hi there,

I was wondering if one of you genius' out there could help me?

I have a sheet (see attached), where i want to input wagon numbers on sheet 1. (There is a named range on sheet 2 called wagonnumbers, used for data validation)

What i don't want is the users of this sheet to input a duplicate wagon number twice in the column.

I can put on normal data validation list, but that doesn't prevent duplicated entries.

I also figured out that i can prevent duplicate entries of wagons using data validation custom and typing in a Countif if formula, but that doesn't use the list drop downs that i desire.

I am looking at making the sheet as simple to use for the end user and don't really want to go down the VB macro route if possible.

In a nutshell, i'm after a data validation list that prevents duplicate entries, CAN THIS BE DONE???

Cheers Guys