Has it ever happened that two movies using the same (or substantially similar) script(s) have been showing in theaters at the same time?
-FrL-
There were two biographies of Jean Harlow – both names Harlow – released five weeks apart in 1965. In theory, they could have overlapped, but I believe they were both such flops that the first was probably gone by the time the second came out.
Weren’t those two movies about Truman Capote both out at about the same time a couple years ago? They may not have both been top screen at the same time, but I think they were both available in theatres during the same period.
Capote was released in early February of 2006, and Infamous was released in mid-November of that year.
Technically speaking, scripts are intellectual property so they are owned by a specific producer (or studio), who is unlikely to spend twice as much money making two movies which he then subsequently releases simultaneously.
So the same script? I’d venture a No. Of course, there are plenty of examples of similarly themed movies (Antz/A Bug’s Life, Deep Impact/Armageddon, Capote/Infamous etc.) having release dates near each other, and there are many many threads detailing these quasi-coincidences.
Deep Impact (May 1998)and Armageddon (July 1998) came out around the same time, with similarly themed plots.
And not in theaters, but there were I think three versions of the Amy Fisher story as made for tv movies.
ETA. “damn you ArchiveGuy” 
Again, not same script but same premise- there were two films about the fictional escapades of a daughter of the U.S. President being developed at the same time, both titled First Daughter. One of them was postponed to not make it seem like a ripoff of the other, and one was renamed Chasing Liberty.
In Japan, Metropolis (an anime movie) and AI (by Steven Spielburg) were both playing at the same time.
They were very similar, though Metropolis succeeded at what it was attempting while as AI didn’t so much. Neither was amazing, though I’d recommend the former for people to watch well enough.
When the original Bela Lugosi *Dracula * was filmed a second Spanish-speaking cast used the sets and costumes during off hours to make a duplicate movie for the Spanish language market.
Yeah, but I figured maybe back when things were still getting figured out this wasn’t such a hard and fast rule. In other words, I thought it possible that in the earlier days of film a single script could be sold to more than one studio or something.
-FrL-
Bingo!
-FrL-
Nope, the practice of purchasing rights to already existing properties (books, plays, etc.) is as old as cinema itself. And back in the studio days, you typically didn’t shop around a script; all screenwriters had contracts with studios so territorial possessiveness was probably even higher back then…
But that doesn’t prevent different studios from adapting the same public domain book or story into a film. The same year Spielberg’s War of the Worlds was released two different studios released low budget versions straight-to-video. One of which was actually set in Victorian England (granted it looked like total crap).
**Volcano/Dante’s Inferno
Dangerous Liaisons/Valmont
Aladdin/The Thief and the Cobbler
Never on Sunday/Breakfast at Tiffany’s **(IIRC, both of these even opened with the female lead singing the theme song, which went on to become a massive international hit)
Mean Girls/Saved
A View to a Kill/Never Say Never Again
None of these pairs were simultaneous, but each was released within a year of the other.
The first of these two comparisons, though apt, is a little unfair- The Thief and the Cobbler was in production for over 30 years. When it was releases, Disneyesque musical numbers were added and the title was changed to Arabian Knight, making it seem like a poor ripoff of Aladdin (at least that’s what I thought when I saw the ads).
And A View To A Kill and Never Say Never Again don’t really have much in common other than they’re about James Bond (in fact, Never was a remake of Thunderball by one of the writers of that film who claimed to have invented the movie Bond). As such, You Only Live Twice and Casino Royale could also be listed, even though the latter was a parody of Bond movies.
The Lugosi Spanish-language Dracula was SOP in the early 30s. Laurel and Hardy made movies for the Spanish market the same way: reshooting them with Spanish dialog (which they spoke phonetically, which made them even funnier, since it came across as though they had some bizarre comic accent).
And I thought Morgan Freeman’s POTUS character delivered a line about “There will be no armageddon.” 
Nitpick: it was actually Octopussy and Never Say Never Again which were both released within a few months of each other in 1983. AVTAK was the only Bond film released in 1985.
Sir Rhosis
Would you count re-releases? Many of the early sound films were shot-for-shot remakes of old silent films. Many of the early color films were shot-for-shot (and often word-for-word) remakes of old black-and-white films. While the newer film was playing in the prestige theaters, I would be highly surprised if the studio did not have the older version playing the “grindhouse” circuit.
I do know of anything specific to the OP, but I have read that in 1952, when the Flash Gordon TV show was on the air, Universal took the old Buster Crabbe serials and re-released them. They would take twelve 15-minute episodes, edit them down to a single 2-hour episode, and release that to the theaters.
If they would do that for a cheesy kid’s TV show, they would surely do that for something like Beau Geste or The Prisoner of Zenda.