I've imported a data file into Excel that has a unique 'key' value in column
A and a list of associated values in Column B. The number of values in
column B will vary and the 'key' value in column A only appears once per set
of associated Column B values. I need to fill in the blank cells in column A
with the 'key' value so that each row will have a values in A and B.

This is too large a file to do manually with a manual fill-down each time a
new value in column A is encountered. Any suggestions or solutions?