回复：移民中介公司说的，说双程容易被海关怀疑没诚意，拒绝入境

Canadians are not so stupid. I think Canadian Customs Officers know many Chinese immigrants just come to land and then go, especially those from the US. But Canada still let them in because no laws say you must stay for some time after landing. The worry is not necessary.
Anyway it seems you do not care much about the Canadian immigration. So in case you are refused, you do not lose much.