I don't know if this is possible:

I would like apply data validation to a range (single column) so that the
items in the drop-down list are generated from any items already entered into
that same range. Kind of like "type ahead" except with data validation. So
upon first use the drop-down shows no data, but then after the first entry
into that range, that entry becomes available in the drop-down, after two
entries they are both available in the drop-down, etc. BUT the items in the
drop-down should be unique: if I have entered (selected) the same text 50
times, I only want it once in the drop-down. Is that possible - without
being too complicated?

TIA.