Im late as always but i have few of 12706 modules and they range from 2,5 to 2,7Ω and one broken(fractured) sits at 200+Ω at 23º Celsius, im not sure if they "toast" as i have never broken one by using it without or using just one heatsink, resistance increases when you heat them but they should never go up to 120-150Ω, high resistance is a indicator of mechanical fracturing of the semiconductor pellets or broken soldering joint, i don't have the equipment to test them further at the moment but i can safely say that your Peltier is broken and your measurements are right, my conclusion is that you just had a bad product that broke after some usage, you could get more powerful one and limit your PSU's current if you want it to last long.