I went through this recently, and got a lot of help from the folks here. On my next-to-useless extended warranty from HD, they finally sent a guy out who knew exactly what the problem is,......common , design defect.... Within about 5 minutes he had removed the duckbill which was all gummed up and stuck shut, and left us with instructions to turn the unit OFF for 24 hours to defrost the drain tube area. Thanks a lot, bubba.
Anyway, the tech sheet referenced within this thread gives you the # for a new duckbill, about $4. The folks on the board here explained that I may have frost issues in the freezer if I don't replace it. I intend to, but have not done it yet and after 2 months all is still well. We actually probably don't open the freezer door 4 times a week so that may be why we are skating by right now!
Anyway, the basic issue is the high temperature near where the duckbill is located deteriorates the rubber so the slot sticks closed over. Some has suggested also a light coating of silicone oil on the new one.