My first column is a list of surnames so non-unique data. My second is a list
of first names, again non-unique but there is no duplication of the
combination of surnames and first names. I need to list all the first names
which go with an individual surname.

eg
Brown John
Brown Peter
Brown Michael
Smith John
Smith David
Jones Peter
Jones David

(but about 500 rows altogether) and I need to be able to, using the above
example, enter Smith in a cell and get a list showing "John" and "David", or
enter Jones and get "Peter" and "David"

Can someone help? Please?