Duplicate values in the 2nd parameter (“keys”) are not a problem.
The 1st and 2nd paramters need to evaluate to the same number of values
The 2nd and 3rd parameters need to evaluate to the same number of records
Formulas that use LOOKUP cannot be sorted
LOOKUP looks into a long list of values (input); finds a match in a second list of values (keys), and, where there is a match, returns a corresponding value (values).
LOOKUP looks into data with duplicate values (input); deduplicates the input records (keys), and returns a summarized value based on the deduplicated data.