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Dave pulls stupid trick on wife in Caribbean

And the No. 1 reason David Letterman is a cad: The “Late Show” host brought the assistant he was sleeping with on Caribbean vacations with his wife and their young son, sources told The Post.

Letterman’s wife, Regina Lasko, had no clue the comic was having an affair with his pretty, much younger assistant Stephanie Birkitt — a $200,000-a-year employee — when she was tagging along on the cozy, romantic trips, sources said.

Lasko, 48, “has to be really, really pissed” to learn recently that Letterman was so brazen as to have invited his paramour on vacations with their 6-year-old son, Harry, a source said.

At least one jaunt was to the Caribbean island of St. Barts.

Lasko — a former show staffer herself who married Letterman in March after 23 years as his girlfriend — believed that Birkitt was only an innocent “friend” and “co-worker” at the time. Letterman also brought other friends and co-workers along for the ride.

The Post previously revealed that Birkitt, 34, and Letterman, 62, took hikes together last fall at his eastern Montana ranch.

Birkitt, a longtime “Late Show” staffer, was identified as Letterman’s lover last week when her ex-boyfriend, CBS News journalist Robert “Joe” Halderman, was arrested for allegedly blackmailing the comic with threats to reveal his dalliance with Birkitt.

Halderman, 51, had first learned of the on-again affair in December, when he read Birkitt’s diaries. They decided to stay together after Birkitt vowed to stop having sex with the multimillionaire TV funnyman.

But Halderman, a hot-shot “48 Hours” producer, went ballistic and dumped Birkitt after spotting her and Letterman making out in a parked car in the driveway of Halderman’s Norwalk, Conn., home, where she, too, lived.

“He sees the two of them . . . in a very passionate embrace,” said Halderman’s buddy, Dr. Bob Arnot, on ABC’s “Good Morning America” yesterday.

“He is beside himself. He really believed that he was being played; that Joe is the ‘backup guy,’ ” said Arnot, citing Halderman’s own claims to him. “At this point, he’s just enraged.”

Sources earlier told The Post that a jealousy-driven Halderman allegedly cooked up the extortion scheme to get “revenge” on his rival for Birkitt’s affections.

“This wasn’t about money,” said a source. “It was about making Letterman miserable . . . He wants to hurt Letterman as much as he can — and he wanted to hurt the girl, too.”

At 6 a.m. on Sept. 9, Halderman allegedly handed Letterman’s limo driver a package containing a screenplay outline about a philandering talk-show host and excerpts from Birkitt’s diaries about sex with Letterman.

Sources said Halderman indicated he wanted to hear back from Letterman by 8 a.m. that same day. At 8:30 a.m., sources said, Halderman attended a memorial service for CBS anchorman Walter Cronkite.

On Oct. 1, Letterman stunningly disclosed on his “Late Show” that he had had sex with multiple show staffers, and had been the target of a blackmail plot.

On Monday, Letterman again mentioned the scandal on air, apologizing to Lasko and saying she had been “horribly hurt by my behavior” and that the affairs “are in the past.”

FoxNews.com reported that Letterman made the mea culpa at Lasko’s insistence. It “wasn’t something Dave wanted to do at all,” a source said.

“He wanted the whole thing to blow over,” said the source, who is close to Lasko. “But Regina told him he better make an apology to let everyone know how sorry he was for how much he hurt and humiliated her.”

FoxNews.com said Lasko demanded Letterman go over with her what he was planning to say, and she was willing to allow him to make “one or two jokes . . . as long as they weren’t at her expense.”

But Lasko “was not happy at all” when Letterman made additional remarks, and “felt like Dave’s humorous take on the apology lessened the sincerity of it,” the source said.

larry.celona@nypost.com