They would eat a lot
You could expect these people to need many more calories than a normal human. It takes energy to start and stop the additional mass, even if the enhanced muscle ability make the movement look completely normal. Additionally, even at rest, muscle take more calories to maintain than other tissues in the body. I imagine the enhanced muscle would need a similarly enhanced flow of calories in upkeep.
Net result: these people probably spend much more time eating than others, or possibly would only eat calorie dense foods
Note: a similar effect might be found with their breathing. Even a professionally athlete is going to get tired and breathe heavily if they carry around another person's weight all the time. Lung capacity / efficiency might increase to compensate, but if it plays out like this, I think it unlikely that these people would spend much, if any, time at high elevation where the air is thinner.
Abnormal body temperature
Or at least, that's how it would likely feel. I'm interpreting twice as dense to mean that there's literally twice as much physical stuff in the same space. This would mean there is effectively twice as much surface area for purposes of thermal transfer.
Net result: with any physical contact (say, shaking hands, for instance) both parties would feel any temperature differences much more acutely. It's possible these people would have constant hot/cold hand syndrome, or at least be perceived that way by normal people.
No static electricity(possibly)
Higher material density decrease electrical conductivity, all other thing bring equal. (I'm using compression of air as a logical reference point, so this might not even apply since people are made of solids and incompressible fluids). These people would effectively be electrically insulated.
Net result: tasers, Van der Graff generators, etc might not work as well on these people. It could also protect them from electrical magic you have them wield, if that's a thing in your world.
They don't get sick
Traditional diseases rely on germs bring suspended in a person's blood or cells. If these people are twice as dense, this simply would not happen. It's like the swimming problem in the other answers, except in reverse. Normal diseases would simply float and collect in certain points of the body where they would be quickly eliminated. If magical traits are at all genetic, you could expect their immune system to eventual center around (or even create) these points. Perhaps the magical community have their own unique set of diseases that normal people can't get?
This one is pretty simple. Heavy people would displace more dirt/mud when walking
If everything in their bodies is twice as dense, it's fair to assume that their... excrement would be as well. This would create a problem if they needed to use modern plumbing, as it's entirely possible moving water would simply not have enough mass to clear a toilet bowl.
Note: hair is interesting to consider as a non living component of a person. Either the hair is just as strong as normal (limiting it's length due to the extra weight) or it doubles in strength like everything else so it would look sort of normal. Sort of, because if it's denser, it wouldn't blow in the wind as much as a normal person's. It's possible normal people would want to use it for making rope, given its strength. Or if it is just denser but not stronger, normal people might grow their own hair past that point to prove, immediately and visually, that they are not magical (useful for a diplomat, say for instance, since the couldn't then be a magical assassin)
I'm sure there's more, but that's all I can think of off the top of my head. Cheers!