Hi José,
Normally, you'd make extra queries (one for each combo box) that does a SELECT DISTINCT on the attribute you need, but since this is external data, this may not be possible (I can't judge). Depending on how many different values you have, and depending on the size of your list, there are several approaches you can take. The easiest would be to ForEach the list and check the values, adding the unique ones to a list. But that may be slow depending on size of the list (you may want to use a TextDictionary to test for doubles more quickly). Also, the SortRecordList extension has a "remove duplicates" feature, but I'm not sure whether you can use that on a specific attribute, or only on the entire record.
Normally, you'd make extra queries (one for each combo box) that does a SELECT DISTINCT on the attribute you need, but since this is external data, this may not be possible (I can't judge). Depending on how many different values you have, and depending on the size of your list, there are several approaches you can take. The easiest would be to ForEach the list and check the values, adding the unique ones to a list. But that may be slow depending on size of the list (you may want to use a TextDictionary to test for doubles more quickly). Also, the SortRecordList extension has a "remove duplicates" feature, but I'm not sure whether you can use that on a specific attribute, or only on the entire record.