Basic Trope: A character has the ability to duplicate the powers of an entire group of people simultaneously.
Straight: The Assimilator, a frequent foe of the Justice Team, has the ability to duplicate the powers of all the members of the team who are present this issue combined.
After the Assimilator gets a power upgrade, he's able to use the combined powers of anyone who has ever been a member of the Justice Team, even if he hasn't actually met them.
The Assimilator is capable of using the combined powers of all members of the Justice Team past, present, future, alternate realities, etc.
Downplayed: The Assimilator can only copy and combine a relatively limited set of powers, for example "mutant powers." So he's very powerful if the Justice Team line-up is all mutants, but severely underpowered against a team consisting of a Squishy Wizard, a Badass Normal and a Hollywood Cyborg.
Copying other people's powers is explicitly the Assimilator's power set.
Alternatively, Stopgap's powers can only manifest if nobody else nearby has the same power. Powerlift shows up, Stopgap loses his Super Strength.
One of the Justice Team members is actually Blessed with Suck. Since the Assimilator isn't wearing the Applied Phlebotinum costume that mitigates the suck, the villain becomes helpless.
The Assimilator is only able to copy one of each opponent's powers at a time. Copying Powerlift's Super Strength allows him to lift the car he's trapped under, but he injures himself in the process because he does not have the Required Secondary Powers.
The Assimilator anticipated that happening, and made sure to get a copy of the Applied Phlebotinum—he just hadn't turned it on yet.
The Assimilator realizes the problem before he strains himself, and is careful to absorb Diamond Guy's Nigh Invulnerable body structure as well before doing any heavy lifting.
The Assimilator fights the Silly Six—and winds up duplicating their combined ineptness and Chew Toy status.
Zig Zagged: Assimilator gains the combined powers of the Justice Team, but it was a trick created by his illusion ability, but that illusion ability was duplicated from Image Master, and it turns out that Assimilator is actually using the combined powers of his criminal allies.
Averted: There are no characters in the story that can duplicate other people's abilities.
Enforced: "We need a really epic villain for Justice Team #100." "How about a guy who has all the powers of the Justice Team, combined?" "Sounds good. Like the way your voice dropped an octave just then."
Lampshaded: "I would never have been able to defeat the Justice Team...if I didn't have all their powers combined!"
Invoked: The Assimilator, back when he was an ordinary Mad Scientist, acknowledged that superpowers are the tools to win a fight, and developed a Super Serum to give him just such abilities.
Exploited: The Assimilator creates a Mass Super-Empowering Event, then secretly guides those who have gained powers he wants to the Justice Team so he'll have use of their abilities.
"Nice try, Assimilator, but we're all wearing jamming devices that prevent you from duplicating our powers."
Discussed: "The Assimilator is one of our toughest foes, thanks to his ability to duplicate and combine our own powers."
Conversed: "Wouldn't it be cool if you had all the powers of the Justice Team combined? It would be like winning the Superpower Lottery!" "Umm, would that include Amazing Girl's Most Common Superpower? 'Cause I'm a guy."
Each time the Assimilator duplicates someone's powers, he also copies a bit of their personality into his own. Over time, he has grown to be less and less himself, and more a conglomeration of other people's personas, such that he's not even sure what his real name was any more. This leads to an epic Villainous Breakdown.
Alternatively: The Assimilator may have their combined powers, but the Justice Team still has experience and the sheer weight of numbers on their side. Armed only with powers he barely understands and outnumbered severely, the Assimilator quickly finds himself in over his head.
The Assimilator absorbs the powers of someone with very dangerous powers, and his inability to control them results in him suffering a Superpower Meltdown.
While recovering from his breakdown, the Assimilator realizes that if he dumps the power, that bit of personality goes away too. After "clearing out the attic", the Assimilator returns to action, now knowing that he mustn't hold on to any given power too long.
After his first embarrassing showing, the Assimilator practices his powers, learning how to use them to their full potential, and even to use the powers of two different members at once to perform feats no member of the Justice Team can match. Setting his sights on a "divide and conquer" strategy, he quickly brings the Justice Team to the brink of annihilation and earns a reputation as one of the toughest foes they've ever faced.
He uses Copy Lad's powers to copy himself creating an army of All Your Powers Combined soldiers, only each duplicate keeps copying each others powers.
Played For Drama:
The Assimilator retains copied powers for five minutes after he leaves their presence. He fights the Justice Team just long enough to disable SplitSecond with Blizzard's powers, then runs away to commit as many crimes as possible before he loses their powers.