I would advise against it because your new shingles comes with a warranty....by putting them on top of the existing shingles would kill the warranty....& also would be alot of weight on the suport beams. My husband use to roof for a living & had his own company....he NEVER layed shingles atop old ones.
GL with the job : )

Thanks for your input.

I've done it both ways, but always had the owner sign a waiver stating that there was no warranty for the job if i shingled over the old shingles. I would rather remove the old shingles so the decking can be inspected for any damage. I think some locales require them to be removed.

do not do it..have you ever lifted a package of shingles??? they are very heavy..and they will not seal properly over old..

i have seen people do this and it is a huge mistake..

Thanks to everyone for your insight. We removed all the old shingles before replacing, which seemed to be the right thing to do.