I tend to think that Groucho, wealthy and middle aged, was no longer really interested in proving anything in movies. His energies concentrated wonderfully on the radio and then television quiz show YOU BET YOUR LIFE. And he was right - his stardom remained high as a result. For that matter neither Harpo or Chico really needed to prove anything about their talents either. LOVE HAPPY was shot because (like A NIGHT IN CASABLANCA) Chico had some money troubles tied to his gambling. But Harpo and Groucho forced Chico to curtail some of his spending habits. From the point of making movies to impress none of them needed it.
So I suspect Groucho was less than really choosy in picking his films. He definitely lucked out in DOUBLE DYNAMITE by having a film with Frank Sinatra and Jane Russell. Both had followings (Sinatra's first following peaking in 1948; Russell being notorious for THE OUTLAW). But the story is a rather weak one. Sinatra is a clerk in a business owned by Howard Freeman. He manages to save Nestor Paiva's life, and the grateful bookie puts money down for Sinatra on a sure-thing horse that wins big. Now Sinatra has money to burn, but just then there is an audit of the books, and Freeman discovers a huge discrepancy. So Sinatra becomes his chief suspect for embezzlement.
Sinatra's problems are that he can't prove Paiva has repaid like this, and even if he could Freeman tends not to believe him. His only allies are his girlfriend and fellow worker Russell, and his closest friend Groucho (as Emil J. Ketch, a philosophical waiter with some biting wit). In the plot of the film, Groucho tries to help Sinatra by trying to get information from Freeman that may lead to the actual embezzler. This leads to the best portions of the film.
I have commented on Howard Freeman elsewhere on this board. A gifted character actor, he is unjustly forgotten today. He was capable of dramatic performances (he was a memorable Himmler in HITLER'S MADMAN, and he was the crooked landlord of the fleabag hotel that Alan Ladd used in THE BLUE DAHLIA), and he was equally good in comedy (he is the wealthy sausage manufacturer who is convinced by an intruding Stewart Grainger not to marry the devious Eleanor Parker in SCARAMOUCHE). He was also one of the few character actors who ever had a chance to reverse a dramatic performance into a comic gem, when he took his self-centered, fatuous Himmler and used it as a nice guy mistaken for a Himmler in a CAR 54 WHERE ARE YOU? episode. With his abilities he was a first rate foil for Groucho.
Freeman's Mr. Pulsifer is not fully prepared to prosecute Sinatra, but he certainly makes Sinatra aware of his peril. So Emil decides that he has to get to know Pulsifer and pry out of him various information about other potential suspects (such as Pulsifer's son). How to do this? Well Pulsifer has not met Emil as a waiter, so Emil dresses up as a wealthy potential business investor. Sinatra is footing the bill for Emil's luxury rooms at a hotel, and his wining and dining Freeman. Of course, the fatuous Freeman does fall for it. Here's the type of man he wants to associate with: a real man of the world. While Groucho spins the most outlandish lies, Freeman readily, greedily swallows them. The scenes between them are quite good.
Groucho does recite "Gather ye rosebuds" at one point, and he has a nice duet with Sinatra. So there are positive points in the film. Ms Russell does the best she can but her lines are not memorable (the title's double entendre is the limit of wit regarding her role). It is a pleasant film, at times almost rising with Groucho and Freeman, but it is not up to the best work Groucho ever did.