"We're running about 10 minutes behind," Randy Zeibert, the owner's son and partner, apologized as the backlog of arrivals reached gridlock in the foyer of the new Duke Zeibert's last night. "I'm afraid it will be 10 or 15 minutes before we can show you to your tables. Why don't you wait in the bar?"

Several customers were seeking to follow this advice when former Maryland governor Marvin Mandel breezed through, intermingling "Hellos" and "Howahyas" with the outgoing puffs of smoke from his pipe, and--to the tune of "Right this way, governor!"--was whisked directly to his table.

It was business as usual on the corner of Connecticut and L, just as if the last three years had never happened. Like Douglas MacArthur, Duke Zeibert had returned.

After 39 detested months of commuting among the race tracks and golf courses of Florida and California, Washington's most conspicuous restaurateur--purveyor of matzo balls and strawberry cheesecake to the famous and the not-so-famous--was host of honor at his own gala unretirement party. He was back on the very corner from which he had been expelled by the wrecker's ball in May 1980--"the best corner in Washington," as he said then and as he reiterated yesterday.

Retirement was "the biggest bore of my life," said Zeibert, describing himself as "72 years young and my heroes are George Burns and Bob Hope."

"I didn't come back in the business because I was broke," he said. "I came back because I miss people. I miss guys like Edward Bennett Williams, Bob Strauss, Tip O'Neill, Buchwald. Now I wake up in the morning and I'm excited again. I'm going to have a purpose. Hey, I've been a competitor, I've been in action, all my life."

Speaking of competition, Zeibert was trying to adopt a live-and-let-live philosophy when it came to his former aide and future competitor Mel Krupin, who has set up shop half a block north on Connecticut Avenue, recreating the old menu down to the last pickle.

Zeibert was trying--but failing.

"He's just a clone," he said. "Can you believe what he's saying? His menu. His staff. If he was so good, why did so many of them want to come back here?"

"He didn't even know what a kitchen was when he came to work for you," volunteered Benny Berman, a veteran Zeibert waiter.

"Listen," said Zeibert. "Let's . . ."

"Let's forget about it?" Berman said.

"That's right," said Zeibert. "I'm not going to worry about him. To me it's no contest, and I'm not being smart. I'm a humble man."

The decor at the new Duke's--on the second floor of the Washington Square complex built by developers Theodore N. Lerner and Albert Abramson--is decidedly more chic than at his old sidewalk-level quarters in the defunct La Salle building. The traditional Zeibert colors of brown and blue have given way to a subtler mix of burgundy, brown and khaki green against a background of mahogany woodwork, satin bronze trim and etched glass.

But the boiled beef and chicken, the crab cakes, the celebrities from the world of politics and sports, and above all Duke Zeibert himself, seemed none the worse for wear.

"Hey, Duke don't change," said Duke. "Duke is the same Duke. You don't change the spots."

"Thomas Wolfe said they never go home again," said Edward Bennett Williams, principal owner of the Baltimore Orioles, from his ringside table. "He was wrong. We're home."

Taking a seat beside him, Zeibert asked Williams if he missed the old design scheme. "Yeah," Williams replied, "but I like missing it."

Redskins owner Jack Kent Cooke was also on hand, for the ceremonial purposes of depositing his team's Super Bowl trophy in Zeibert's vestibule--"for a reasonable length of time," he said.

"There's been a lot of controversy about Mel Krupin and me," said Cooke, alluding to Krupin's recent decision to fly the banner of the new Washington Federals on his premises. "I want you to know that I'm playing it right down the middle. I parked my car at Mel Krupin's tonight."

When Williams tentatively offered to place a World Series trophy at Duke's, too--provided the Orioles come away with it this fall--Zeibert was ecstatic.

"We'll have the Orioles and the Redskins and Duke," he said.