OK to cut double top plate to clear exhaust duct?
Do not cut the double top plate. 

A double top plate acts as part of a giant horizontal beam that resists wind and seismic loading. Cutting the top plate is the same as notching a regular beam holding a floor up. 

Having it in the corner of your house is good, but I wouldn’t do it until a professional could check it out. 

Btw, the Code requires continuous double top plates lapped at the corners and lapped a minimum of 48” elsewhere with a minimum of 8- 16d into each side. of the joint. (See ICC 2308.5.3.2.)