I have a large table of items for sales in many states. The two letter state abbreviation for that particular client is in a column. The clients are listed in order of the date of the transaction, so many states are duplicated quite a few times. My CEO wants me to create a cell that lists the states where we currently have active clients. I have done this via the following formula:

=TEXTJOIN(",",TRUE,B10:B500)

This works, but includes duplicates of states. For example, it's returning the result: AZ,CA,FL,TX,FL,CA,CA,KY, etc. Is there any way to have it list the states, without the duplicates?
Or is there another way entirely to accomplish this? Help!