New to CTE - How to do a count after making table unique
Don't know why you need a CTE (or a subselect), wouldn't this simply be

select person, date_, max(where_) from tbl1
group by person, date_
having count(distinct where_) &lt; 2


(fiddle)

PS. Note I changed the table definition to 1) remove backticks that are only used by MySQL to quote identifiers (Redshift is based on Postgres) and 2) rename columns with names that conflict with SQL keywords.