How can I display only unique records in a lookup drop down?

You can display unique records using the distinct() Deluge function. This function returns the distinct values of a field 
in a form.

For example, assume the field "ProgramCodes" is a lookup to the "ProgrameName" field of the "Program" form. The sample code 
given below is used to populate only unique values to this field.If you need search criteria on "Programs" put them in 
the brackets instead of [ID != 0].

1. clear ProgramCodes;
2. ProgramCodes:ui.add(Programs[ID != 0].distinct(ProgramName));

Refer the help documentation here.