My theory is that ragular usage is the best preventative for this kind of problem. In the winter in the UK the aircon does a good job as a dehumidifier and keeps condensation and damp out of the car. I think it is lack of use, possibly spiders, and drying out in hot weather, that "welds" up the ducksbill.

I believe that if you drive the car for at least 10 miles 3 times per week, every week of the year, you will not get this problem.

 

The duckbill was changed starting with the 5.0 models to a design less susceptible to blockage. Some of the early 5.0 cars still have the old model drain though.

 

I agree. However:
The original duckbill design eventually failed for two or more reasons:
1. The flexible rubber became less flexible and/or adhered to itself, reducing the effective opening.
2. Accumulated condensate, no matter how little, plus dust, pollen and what have you, forms an excellent medium for growing mold and mildew.
The mold and mildew then complete blockage of the condensate drain.

If the duckbill has been cut or otherwise modified as recommended by the many threads/posts on this subject, but still doesn't drain properly or frequently recurs, a biological issue is likely.