116 minutes. PG-13 (for violence and action throughout, and brief strong language)

It’s every Hollywood star’s worst nightmare — that there’s someone younger, prettier and — oh, sweet Jesus — less expensive coming up behind you to take your job.

If you don’t think “All About Eve” was a documentary, you’ve never dated an actor.

That classic show-business paranoia is the subtext that drives “Gemini Man,” an action flick with a twist.

Henry Brogan (Will Smith) is a 51-year-old former hit man for a CIA-like government organization who’s just trying to enjoy a crisp, refreshing Coca-Cola in retirement. (Sorry, the product placement in this film is as subtle as one of Big Willie’s rhymes.)

But when he discovers something he shouldn’t have about his final job, his former boss (Clive Owen) sends his best assassin to kill him.

That assassin turns out to be a 23-year-old clone of Brogan himself, played by Smith with the help of de-aging face technology.

It’s like “Bad Boys” Will Smith squaring off against “Bad Boys For Life” Will Smith, like the Fresh Prince versus the Stale King.

Smith’s character, joined by another agent (Mary Elizabeth Winstead), flees from the young whippersnapper through the Georgia swamps and the Budapest underground, fearful of losing his life or, worse, magazine covers or his lucrative points on the back end.

The film is directed by Ang Lee, and “Gemini Man” shows occasional flashes of the eye-catching action that made 2000’s “Crouching Tiger, Hidden Dragon” a hit, including a propulsive motorcycle chase through the streets of Cartagena, Colombia.

Unfortunately, the rest of the film is closer to Lee’s “Hulk,” an often-draggy, muddled misfire.

Much of the blame can be placed on the script, credited to so many people, the title should have been changed to “Gemini Men.” It’s not all that smart or twisty and might have been more expedient to have it star Dolph Lundgren and go straight to Redbox.

Too bad. The idea of facing off against your younger self is an intriguing one, but the movie never does anything particularly interesting with it. When Smith 51 and Smith 23 first cross paths, neither thinks much of the resemblance. Maybe they previously met at a Church of Scientology mixer?

The writers’ one stab at emotional resonance comes in exploring the relationship between the clone and the old boss as his surrogate father. But that also falls flat.

As with Lee’s 2016 “Billy Lynn’s Long Halftime Walk,” the director shot “Gemini Man” with a high frame rate designed to provide a smoother, more detailed picture, although some critics have complained it makes movies look like soap operas. (The Post only screened the standard version.)

And isn’t daytime television the final resting place for every actor who’s lost the battle to the young upstarts?

“Hi, this is Will Smith for reverse mortgages.”