An unusual variation on "Duck Variations" was played last week at the West End Theater, where David Mamet's one-act play is on a bill with its Mamet mate, "Sexual Perversity in Chicago."

The two-character play was enacted by one actor, Mike Kellin, who reordered the other role and talked back to his own voice on the recording for five performances. This solo flight was necessary after two consecutive actors cast opposite Kellin were dismissed and after Kellin, at one performance, had even dispensed with "Duck Variations" and performed a monologue about roosters.

None of this confusion was reflected in opening night reviews because Michael Egan, who created the role in New York, was imported for the first night after actor Jack Hollander was judged not fit to perform.

Egan had other commitments in New York, however, until this week, so after another actor struck out early last week. Kellin was forced to go it alone. Egan finally joined the cast two days ago and is expected to be here through the play's scheduled closing on June 26.

Anyone who kept a ticket stub from any of the one-actor performances is eligible to see the show now as it should be seen, with two actors, for no extra charge. Twice the cast for the price of one, you might say. On the other hand, you might say that the whole mass is no way to run a theater.

Extensions: "A Midsummer Night's Dream" at the Folger through July 10, making it the longest Folger run ever; "Canticle" at the New Playwrights' Theater through June 12.

Howard Witt, one of Arena Stage's most popular and familiar faces for nine seasons, will leave town after "A History of the American Film" closes to try his luck in Hollywood. At first he'll camp out in the house of his old pal Ned Beatty, another Arena alumnus who has lately become one of America's most ubiquitous screen actors. If Witt can follow in Beatty's footsteps, he'll be a rich man. "American Film" is Witt's 50th show at Arena.

Since we're flipping through Arena's old yearbooks, we might as well note that Jane Alexander will open at the Kennedy Center Saturday in "The Master Builder," that last year's graduate Max Wright is in "The Basic Training of Pavlo Hummel" on Broadway with Al Pacino after a stint in Andrei Serban's controversial 'Cherry Orchard" at Lincoln Center, and that Wright's classmate Dianne Wiest is about ot take over a whopper of a role in "Ashes" at Joseph Papp's Public Theater . . . Meanwhile, back at the campus, the Arena auction begins at 4:30 Sunday.

A new play by Miguel ("Short Eyes") Pinero opens Friday night at the Back Alley Playhouse. "The Sun Always Shines for the Cool" was written three years ago for the ambitious series of original American dramas that Joseph Papp planned for Broadway, but the series was cancelled before it began. Pinero was impressed by the Black Alley production of "Short Eyes" this year and offered "The Sun Always Shines" to the Kennedy Street playhouse. It will play through July 10, Thursdays through Sundays . . . Back Alley's performances of "Gandiji" at its downtown Studio Saturday and Sundays evenings will benefit the Julius Hobson Associates.