how to find unique non matches?

[Blewis21]

I have a list of around 500k rows and I'm trying to fill in blanks under 'code' with unique values assoiated with the ID (product) 

my picture shows product 1 will a null value and I need to fill it in with a new code, but it can't be a code already used by product 1. so my options are either ABC or GGG. I only need one value (ABC or GGG) and it doesn't mater which one. how can I do this? 

[binuacs]

@Blewis21 This can be done with the help of the Multi-Row Formula tool

1. Select the Update Existing Field option and select the field 'Code' from the dropdown

2. Select NULL from the dropdown Values for Rows that dont exist

3. update the expression IIF(isNull([Code]),'abc',[Code])