Once was not enough.

The Bullets beat the league-leading Philadelphia 76ers for the second time in seven days, 129-108, last night before 19,035, the first sellout crowd this season at Capital Centre.

"I'd have to say it was a '10'" said Kevin Grevey, the Bullets' leading scorer with 31 points. "Beating Philadelphia is not like beating anyone else."

The Bullets defeated the Sixers in Philadelphia last Friday, 104-102. Last night's defeat was the Sixers' worst of the season.

The Bullets are only the second team to beat Philadelphia twice this season. The New York Knicks, whom Washington plays tonight in Madison Square Garden (WDCA-TV-20 at 7:35), are the only other team to have two-timed the Sixers.

The Bullets beat the Sixers at their own game: running. At the end, after the Sixers had been outscored, 28-10, on the fast break, they looked as if they had been run over. Kevin Porter, who did not sit down, or slow down, until midway through the fourth quarter, had 24 points and 15 assists.

"With seven (minutes) to go, I said, 'Are you all right?'" said Bullet Coach Gene Shue. "He looked up at the clock like he felt like saying 'Take me out,' but he didn't. So the next time, I took him out.

"That was, for us, a perfect game. We beat the Sixers the last time. We knew they'd be coming in ready. We felt we had to play a great game, which we did. I don't know how we could have played any better."

Indeed. The Bullets turned the ball over only four times in the first 33 minutes and 11 times all together. "When you run, you have turnovers," said Porter, who has averaged 12.4 assists during the last 18 games. "We've been trying to curb them. When you don't turn the ball over to them, they're not super effective, because they just run up and down the court."

When Porter went to the bench with 6:35 remaining in the fourth quarter, Sixers' Coach Billy Cunningham went to the locker room with his second technical foul. "I deserved the technicals," Cunningham said. "I said, 'We've got to beat (official Bob) Rakel as well as the Bullets. He said, 'I heard you.'"

But Cunningham also said that the Bullets deserved the win. "It was as good a 48 minutes of offense as I've seen all year," he said.

Bernie Bickerstaff, the Bullets' assistant coach, had predicted the Sixers would "come out smoking." But so did the Bullets, including Elvin Hayes, who had 14 first-half points.

The Bullets were behind, 32-27, when Don Collins replaced Greg Ballard at forward. In the next six minutes, with Collins in the game, the Bullets picked up the tempo and took the lead for good.

"We were down by one and went up by five," said Collins, who had three assists, two steals, two points, and one rebound in the six-minute stretch. When he left the game midway through the second quarter, the Bullets were ahead, 47-41.

"I felt good coming in tonight," Collins said. "Gene told me to get in the lane and create some things."

The Bullets led by 70-62 at the end of the half. They were up by as many as 11 in the third quarter with Collins back in the game and the Sixers going 3:01 without scoring a basket. The quarter ended with the same eight-point Bullet lead.

In the first four minutes of the fourth quarter, the Bullets outscored the Sixers, 10-4, to lead, 104-90, and the Sixers never got closer than 14 points. During that spurt, Ballard (23 points) scored six points, and Mitch Kupchak four.

Ballard had a easy lay-off an assist from Kupchak to make it 102-90. Hayes blocked a shot by Clint Richardson, and Ballard scored again on a 19-foot jumper.

"We had to sustain the effort the whole game," said Grevey, who has sustained his effort, scoring more than 20 points in five of the last six games. "You get up 14 but you can't relax. I think that's what they were waiting for, for us to drop our guard. But we were relentless."

The Bullets increased their lead to 24 and the Sixers were chafing, as Julius Erving (a team-high 27 points) and his teammates lost everything including their Chapstick. At one point, the cap to Dr. J's Chapstick rolled forlornly across the floor, just as the Bullets rolled over the Sixers.