I have an Earthway single row planter. Don't know about your Covington. I plant corn, beans, carrots, kohlrabi and beets. I have many different sized plates. Not every plate works best for the type seed it's designated for. Some of the "holes" are too big - some are too small.
Try an alternate plate if you have them. Suggest - something with a smaller hole.
Multiple seeds being picked because pumpkin seeds are so thin. Could be that you will have to plant pumpkins by hand.
Remember - many veggies are designed for multiple planting and then later thinning.