Would you recommend installing new shingles on top of the old existing shingles on a house? What are the pro's and con's in addition to being cheaper...

Cheaper labor is the only pro. Can your roof support another layer? If roof is truss built with 24" OC and OSB, I wouldn't do it. Stick built with 16" OC should be OK. 20 squares with 220# roofing is over 2 TONS of additional weight.

Depending on type of shingles, no more then 3 layers can be put on a stick built roof and no more then 2 on a truss roof.

Chuck