Working with combox and listbox in WPF

Question

Hi all,

I have a combobox with cities and a listbox for customers.

When I select a city, I display all customers who stay there in my listbox. The problem is that when I select a city and the customers get loaded, and I select the same city again it adds the customers again. Is there a way where I can avoid repeating items in a listbox?

I hope this is clear, thanks in advance