going over 1 layer is fine...if their not laminate shingles.
they are double thick and can cause the new ones to sit crappy, not lay flat and seal properly..
if you have square butt or old t-lock shingles, not a prob. in fact t-locks were made to go over top of old shingles, but their pretty much out of production now.
of course it's always nice to remove the old ones, but sometimes there just isn't enough money (besides labor, disposing of old shingles is not cheap, there petro based and their heavy) or whatever other reason. if you go over top, go up in your attic and check the deck, any probs will be visible from below.
also trim your eave shingles (gutter line) back flush with the deck and your rake ends flush too. install your new starter row out a couple inches into the gutter just like before, and run a row of shingles up your rakes hung over an inch. won't get that big bulky ugly look and gives a nice straight edge to cut the shingles to.
synthetic underlay is also way superior and better lasting than using felt (tar paper)
when going over top, ventilation is now critical so your roof won't sweat. don't know what the specs are where you live, but a good rule of thumb is one AF50 roof vent for every 600 sq ft of living space
this is my 40th yr slinging tar and banging shingles, any questions just ask
if you have trusses, no worries about weight. roof trusses are engineered usually for a min. 4000 lb snow load, you will only be adding a couple hundred lbs per square. we actually did a house 'bout 10 yrs ago that had
SIX layers, swear to god that house popped up 2 inches once they were all off