Excel - How to store a value so it does not get used again

[NotSoFastEddie]

The user has to assign a unique value to a cell and make sure it does not get used again, as the input for the row ultimately becomes a unique identifier in a database.

I am not sure if this is something a macro can do or Excel on its on or ... not at all.

[#2]

@NotSoFastEddie I don't understand the picture you included but you can use Data Validation to prevent mutliple copies of the same entry:

so if you try to enter the same value (e.g. 15) it will pop up a warning that it isn't allowed or any custom message you set under the 'Error Alert' tab of that menu.

Notice I use the COUNTIF( [fixed range using $$], [first cell in range] )