Few things:
f you go to a Canadian Consulate and they reject your visa they usually cancel all your other visa stamps on the passport also(just to avoid confusion) (it might vary but this is usual procedure) so you cannot enter US if your visa is rejected.
If you have a H1b stamped on your passport and you are going for restamping Canada is not a bad option I got it done last year and the process was really smooth. I got it done at Vancouver.
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