There are probably multiple ways to achieve what you want, load script based and front end only.
For example, you could create a list box with an expression (select <expression> from field list), then enter as expression:
=aggr(if(count([Property ref.])>1,[Customer Number]),[Customer Number])
Or create a list box for field [Customer Number], then add an expression (from expression tab):
I noticed that my suggested expression won't work as provided (using just the customer as list box field).
So either create a flag in the script as suggested above, or do something similar in the front end, if needed:
=aggr(nodistinct if(count( PartNr)>1,'Multiple','Once'),PartNr)
Unfortunately, you can't search the expression values in a list box, but you can use a straight table with dimension Customer and above expression and make the expression searchable. But a straight table has some limitations and differences compared to a list box.
In a list box, you could create a combined Customer / Flag string to show and select from, maybe something like
=aggr(nodistinct num(CustomerNr,'000') &' - '& if(count( total<PartNr> PartNr)>1,'MANY','ONE'), PartNr,CustomerNr)
If you don't need to be selection sensitive, I would probably go for the script solution.
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