It's quick and easy to check if a particular aerodrome has been uses as a challenge, at least on this MK III version of the thread: Simply go to the top right side of the page you are viewing and click on "Search This Thread". Type in the name of the aerodrome you want to use and/or the associated city. It will then display the post and date, if it has been used. Voila. As this thread goes back more than seven years that's a reasonable cut off -- or perhaps the group could agree on a different timeframe between posting of duplicate aerodromes (3 years, 5 years, whatever...) and a Mod could post a sticky with the rules. If no one wants to assume the task and responsibility of record keeping, a simple search will generally suffice.
Grizzled