The federal government concluded its 33rd annual "Duck Stamp" contest yesterday -- the first in which the Interior Department charged an entry fee. The contest determines the design for the 1983-84 federal migratory game bird stamp, but that usually translates into thousands of dollars worth of reprint sales for the winning artist. The $7.50 stamp, sold at Post Offices, is required to be in the possession of anyone hunting ducks, geese and other migratory birds.

Interior figured that it might as well try to recoup the cost of running the contest, and this year began charging artists $20 to enter. The number of entries dropped 25 percent from last year's record-setting 2,099. But this year's winner, Phil Scholer of Kasson, Minn., said of the fee, "It could have been $100--that wouldn't have stopped me." Scholer, who won on his fifth try, described the contest as "the big thing . . . the very best thing that could happen to someone in my business as far as prestige, visibility and financial reward." His winning design was an acrylic of two pin-tailed ducks.

As for the $31,280 that Interior earned off the event, spokesman David Klinger said it will be used to defray the estimated $50,000 cost of running the event, including a staffer's salary, judges' expenses and the cost of mailing back all the losers designs.