I understand that Joe Blogs will appear 5 times once I have imported all 20 excel spreadsheets into the new FileMaker database.
This is not necessarily the case. It's possible to set up a table that automatically excludes duplicate values during the import process. And there are also other options that can be used--including a looping script that combines the data.
Which of numerous options will work best depends on your data and how it is structured in those excel tables.