HONG KONG yesterday started up the bureaucratic machinery for the forcible repatriation of thousands of boat people to Vietnam.Officials began identifying economic migrants from among Vietnamese refugees who arrived at registration centers yesterday in preparation for their repatriation under an agreement signed between Vietnam and Britain Tuesday. The first forced returns of those determined to be economic migrants could occur in six weeks. Under the terms of an accord signed Oct. 21, Hong Kong next month intends to expel more than 200 "double-backers," Vietnamese who returned to Hong Kong after already going back to Vietnam voluntarily. Washington up until press time had withheld comment on the scheme, although it has categorically opposed the forcible repatriation of refugees who have streamed out of Vietnam since the 1970s. Hong Kong officials and legislators are concerned the plan will provoke outrage overseas. The British territory halted a similar effort in December 1989 because of widespread condemnation abroad. Still authorities have redoubled security around refugee camps and have showed signs of hesitation. "We will attempt to let them return with dignity," says Alistar Asprey, secretary for security. "Whether they do so depends on their own behavior, which we can't control," he says. The launching of the scheme ends several months of fiery debate between those morally opposed to forced repatriation and those who resent the high cost of hosting a ballooning number of asylum seekers. The plan also caps lengthy negotiations with Vietnamese officials who dreaded the high cost of providing a livelihood for their repatriated compatriots. Vietnam will receive about $1,000 from Britain and the European Community for each refugee who returns home. Hanoi also was concerned the program would worsen its estrangement from Washington, which maintains an economic embargo against Vietnam. There are 63,400 asylum seekers in Hong Kong, with 19,700 of them labeled "economic migrants" and 38,800 of them not yet screened.