Twice as many doctors and nurse practitioners than expected took or remained in jobs in Eastern Massachusetts community health centers during the first year of a program that helps pay off their student loans, said proponents of the plan. The program was launched in 2007 with a $5 million grant from Bank of America, and has placed 35 physicians and 12 nurse practitioners at 23 health centers. Most were new hires, but six were current staff members who agreed to stay either two or three more years. The program pays up to $25,000 a year for three years in loan repayments. Recipients agree to work in primary care at a community health center for at least two years.