How to select DISTINCT record to picklist?

How to select DISTINCT record to picklist?

Dear All,

It's ok to add record into the picklist, but how to just add DISTINCT records?

For example:

Here are records in FormA.FieldName:
Item-1
Item-2
Item-1
Item-3
Item-1

I would like just to add Item-1, Item-2, Item-3 into the picklist (the DISTINCT records).

When I used :
type = picklist
values = FormA.FieldName
There are 5 Items in the picklist.

Is there any UNIQUE or DISTINCT function to select record?

Thanks!!