Dynamic removal of duplicates

I have a list of data in two columns. The data ralates to areas and times of cleaning shifts in a hospital environment. The first column is a list of data that indictaes the shifts that are vacant, for example when people are on holidy or are sick. The 2nd column shows the shifts that are subsequently filled by other staff. What I need is for a third column to sort this automatically to leave just the shifts that havent been filled

Workbook attached

Thoughts