Ceran top dual element burner won't do single?

I've got a Ceran (glass) top stove. One of the burners is a dual element, a small round element surrounded by a larger round element.
If I switch this burner to dual, both elements come on, as expected. If I switch this burner to single, I still get both elements on. I had some WARM knuckles before I realized what was happening! This would have broken sometime in the last couple of days.
I haven't started looking into this, as I'm trying to get my turkey done! Am I looking for a big repair here, or something as simple as replacing the control knob?
Stove is a seven(?) year old Whirlpool. Model #GJP84900 Type: 30p
Any suggestions out there?
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Same thing happened to my sister's range last year. Ended up replacing the heating element and wiring harness. That's what the Sears' appliance repair people decided was necessary. Probably could have just replaced the element.
R
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*sigh*
Looks like I have other issues as well. It took FOREVER to cook a turkey the other day, so I picked up an oven thermometer yesterday. I've got a roast in the oven, the temp is set at 500 degrees F, and the oven has been on for almost two hours.
The oven thermometer is sitting around 245 degrees F right now. The thermometer is sitting about 3/4 the way from the top of the oven (near the bottom of the oven). There shouldn't be THAT much difference between the floor and ceiling of an over, should there?
None of the circuit breakers on the stove or in the basement have been tripped, so I'm wondering if the burner and the over problems might be related. Maybe a common ground is loose someplace?
Any ideas where I should start looking? Anyone have a wiring diagram for this beastie?
Stove is a seven(?) year old Whirlpool. Model #GJP84900 Type: 30p
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