Ambroid is not a contact cement -- it is a cellulose or nitro-cellulose based cement or glue where the parts to be joined are put together while the glue is wet.
Duco cement is also a nitro-cellulose based glue or cement. While I found the MSDS for Duco
(
http://www.kelloggmarine.com/msds/DEV- Devcon/DEV_62465_MSDS.pdf ) where the component chemicals are listed, I could not find one for Ambroid, so the solvents in Ambroid may not be the same.
Duco is a very effective glue for some plastics because its solvents dissolve the plastic, and when the solvents evaporate, the plastic is effectively welded together. Very effective at sticking plastic model airplanes together -- creating a joint that is impossible to separate. As a glue for other materials, Duco is applied to one or both surfaces, the items to be glued are put together while the glue is wet, and should stick together when the solvent evaporates. Pretty effective with porous materials, much less so with non-porous stuff, except the plastics that it can dissolve.
If you've used Duco, you know that if you get it on your fingers, it dries quite rapidly, after which you can peel away the thin clear plastic-like material which is effectively water resistant or water proof (probably doesn't meet the current technical standards for "water proof" which involve boiling).
Though I haven't used it that way, I would expect Duco would make a satisfactory canvas repair material.
Though the solvent(s) in Ambroid may differ from those in Duco, the dried glue appears to be much the same -- some form of cellulose. See the short history of Ambroid may be found at
http://www.ottertooth.com/Canoe_pages/ambroid.htm