The return to service of the Three Mile Island Unit 1 reactor helped General Public Utilities Corp. more than double its second-quarter earnings of last year, the company said yesterday.

And, American Can Co., which recently announced the sale of its packaging operation, yesterday reported a slight increase in its second-quarter profit.

General Public, in Parsippany, N.J., recorded second-quarter earnings of $30.1 million.

Earnings per share of common stock were 48 cents, compared with 21 cents in 1985. Second-quarter net income in 1985 was $13 million.

Net income for the six months that ended June 30 was $94.8 million, up from $47.1 million last year. Earnings per share doubled from 75 cents in 1985 to $1.51 in 1986.

Chairman and Chief Executive Officer William G. Kuhns credited the increase in earnings to the reactor's return to service, which allows General Public to charge its customers for the reactor's costs. He also cited a 3.1 percent increase in sales and rate increases granted to the utility's subsidiaries in 1985.

Unit 1 at Three Mile Island in Middletown, Pa., returned to service in October, after being out of service since the accident at Unit 2 in 1979.

Total revenue for the second quarter dropped slightly from $673.3 million in 1985 to $652.2 million in 1986, because of a decrease in energy costs that must be passed along to customers.American Can's net income for the three months ended June 30 was $47.8 million ($1.47 a share) on total revenue of $1.1 billion. That compared with earnings of $45.9 million ($1.52) on $1.01 billion in revenue.

American Can attributed the slow quarterly growth to an extradordinary loss related to debt redemptions, which was offset by the sale of some timberland and a public stock offering of a subsidiary.

American Can is involved primarily in financial services, speciality retailing and packaging.

For the first half of this year, net income was $115.3 million ($3.85), compared with $81.3 million ($2.83) in the same period last year. Revenue totaled $2.1 billion, up from $1.92 billion.