A federal judge sentenced a former member of a top-secret Army unit to one year in prison yesterday for his conviction on charges of stealing $796 while on covert assignment.

"This offense, despite its insubstantial amount . . . warrants imprisonment," because it is evidence of "an abuse of trust imparted" to Lt. Col. Dale E. Duncan by his military status, said District Judge Albert V. Bryan Jr. in Alexandria.

Bryan imposed three one-year terms, to be served concurrently, for two counts of filing false claims and one count of theft. Duncan, who was convicted Feb. 12, faced a maximum penalty of 20 years and $30,000 in fines.

Duncan's attorneys said they would appeal the conviction and Bryan permitted Duncan to remain free on bond pending that decision.

A jury found that Duncan misrepresented to a government auditor that he was owed $796 for a plane ticket when the ticket was actually free because of a frequent-flier program. Duncan testified that he had forgotten that the ticket was free.

The jury acquitted Duncan on two additional counts and Bryan dismissed two other charges.

The charges against Duncan stemmed from a secret, two-year Army investigation of alleged financial improprieties by some of the service's intelligence and special operations units. The investigation stirred controversy among some military officials who contended that it was an overreaction to allegations and that it hurt the effectiveness of some covert Army units.

The probe has also led to court-martial proceedings against Duncan and three other officers, one of whom was charged last week. Lt. Col. Bruce P. Mauldin was accused of dereliction of duty and of stealing almost $15,000 during his work in special operations.

Duncan, a 19-year veteran of Army intelligence work, was employed in the Army's Special Operations Division in the Pentagon when the allegations against him surfaced in the fall of 1983. At the time he was running a secret security operation for other covert Army units under the cover of an Annandale business firm called Business Security International.

In a sentencing memo submitted to the court this week, Assistant U.S. Attorney Theodore S. Greenberg said that 16 years ago Duncan had "demonstrated himself a fraud by accepting a Silver Star for bravery in combat which he did not perform."

Duncan's attorney, John M. Dowd, said of Greenberg's allegation: "As far as we know, they're wrong. We verified [the award] with the 101st Airborne Division."

Last week, Bryan dismissed a suit by Duncan against the Army alleging it violated his privacy by obtaining his credit card records with a court order.