Only Unique Values in Dynamic Drop Downs

I have now managed to make my three Dynamic Drop Downs to work (with some much appreciated help) but now have another issue.

Setup: Select Practice (Dynamic List works fine), select Date (works fine), select Vet - This one works but not fully. What it does is show all Vets associated with a Practice but these are not then ALSO filtered by the Date selection in the previous drop down as I was expecting.

I am probably just getting muddle up in how the database and dynamic drop downs operate but I cannot see my error (i suspect it is with Create Dynamic Array). Once everything selected I need it to show me what Vets are in that Practice on that Date - nothing else. The child and grandchild tables share the same database table.

Also:

How can I return only unique values in the drop downs? I have one drop down based on dates and the same date can repeat in the database but I would like it to only come up once to be able to select it.

Thank you.