There are many different Duet washer models and at least three different engineering designs on them. Citing the model number of your machine would help for getting useful answers.

Your best course if service is needed is to call a local appliance service. No need to go through Whirlpool unless warranty coverage is involved, and maybe not even then if there are local servicers that are factory-authorized warranty agents.

Often spin issues have to do with a restricted drain line, though it could be something else, especially since it works with small loads. If you have one of the older German or Mexican built Duets, there is a removable access panel below the door that gives you access to the pump trap, among other things. If you have one of those models, I would recommend removing it and cleaning the trap. If you have one of the newer Clyde built Duets, accessing the trap requires more involved disassembly, and you should call for service so they can clean the trap for you if needed.

If a repair is needed, I am inferring from your post that you aren't planning on doing it yourself. Appliance technicians, at least good ones, are not parts changing monkeys. Yes, there are some cases where they will know the problem instantly upon hearing a description, but more often, diagnosing is half the battle. In fact, most technicians will not accept a customer diagnosis over the phone before visiting.

The model number is WFW9150WW02. It says it was made in Mexico.

Cool, so you should be able to check the trap. Let us know what (if anything) you find in there.

I'm not sure what the trap is. I took that panel off but I don't know what to look for.

It's a little filter on that twists off the drain pump. You can just unscrew it with your hand. There will be some water inside the pump when you take out the trap so have a bowl or pan ready to catch it.