Our code and code in Texas in general is that a roof can not have more than two layers. When adding a new roof with two layers of shingles on, you must tear off all shingles to the decking and then begin again.
On a "tear off" you take off all shingles, all tar paper, and then replace any decking in need, re paper, re install new turbines, new roof edging, new gutters and then new roofing.
Going over old shingles, comp shingles, its no big deal at all. You start on the down side with the reversed first row, then decide on your design then hit it.
I like the course plan that takes 6 inches off on each row till you get to a six inch tab then start over with a full tab.
Where you need particular attention is the valleys and the straight rows.
Use four nails to each shingle and use a hammer powered by hand. Nail guns sometimes over shoot and tear the shingle.