This has to do with the way runwayfinder resolves locations. There is no way to definitively tell runwayfinder that the location being passed is a zipcode. When I originally write the interface, I used the tag "zip=xxxxx", however this failed on many zipcodes, including the zipcode to my home airport (01602). ZIP=01602 resolves to 1602 Zip Ln, Madrid, IA 50156, USA.
I changed the tag to zipcode to get around this, and it worked well for a while. Unfortunately, runwayfinder is now resolving zipcode to Zipade, Cordesville, SC 29461, USA. So runwayfinder now thinks every flight originates and terminates in Zipade, Cordsville, SC , hence the duplicate message.
If I can get a definitive specification on specifying locations on runwayfinder, I'll probably re-tool the interface to use it. Unless that happens, I simply can't keep re-writing the interface based on guesses.
The best way to contact me is via email:
steve dot foley at gmail dot com