appending a date string only once

I have a date and a folder of zclass instances.

-I want to search through the instances and see if the date matches a date property in the instance.

-If it does not match I want to append the date to a list.

The date may not match many instances. Here's the question. How do append the date only once?

I would like a hint on how to do this with dtml or PythonMethods.