i'm always thinking about the concept of a “clone” in videogame discussions, partly because i've been walking around for about a decade thinking about the claim that there was a “flood of doom clones” after 1993, a phenomenon that, as far as i can tell, simply did not occur.
the concept of a game “clone” used to be incredibly literal. after pong came out, a hundred companies simply made pong, but not called pong, and sold their counterfeits not under the table, but openly, with huge advertising campaigns and the whole bit. this repeated for decades, and unsurprisingly created vast amounts of IP law in its trail.
in some cases, a company like atari would succeed in a lawsuit which established towering new concepts about copyright, then later sue someone else and inadvertently establish more precedent which demolished the one they'd just set. ken williams of sierra once won a suit vs atari, and then on the courthouse steps told reporters that he wasn't sure he actually agreed with the judgment and what it meant about his career and industry. the judgment in his favor, rendering a verdict he requested, i must stress.
there was also the capcom / data east lawsuit, over a decade later, which established that data east was absolutely permitted to create a near-perfect clone of street fighter, because street fighter itself was largely just a series of riffs on cultural tropes. you can read up on that one, it even has a wiki page, and the judgment was juicy and flavorful.
but these were exceptions; clones were overwhelmingly accepted, legal and commonplace behavior through at least the late 80s, and every single successful game got cloned quickly and often crudely. everyone agrees that Power Blade is “a megaman.” so when doom came along, pundits simply assumed it would be massively duplicated, to the point that they didn't really bother checking if that was actually, you know, happening.
you can look into it yourself and draw your own conclusions, but i assert this very strongly: while doom was probably important to the development of the First Person Shooter, it did not actually define a pattern that much else used. if you check the fossil record you will find that very few nearly-identical games were released in doom's wake, and this differs substantially from many other genre-defining works. Jazz Jackrabbit would not exist if CliffyB hadn't been trying to make “sonic, but for the PC,” and the NES had a half-dozen games that were nearly pixel-perfect copies of Zelda, but it's really hard to find a game that looks or plays like Doom.
heretic/hexen/strife don't count - not only do they deviate enormously from the original work, they were based on the same engine. i don't think you can call something a clone when it's a legitimately licensed derivative of the exact same code made in partnership with the creators.
duke nukem 3d could be the most notable “doomlike,” except that it isn't really remotely doom-like. it's hard to identify anything it does that's directly lifted from doom; they figured out that weapons shouldn't be in the center before id did, they added a massive vertical component to the gameplay, non-abstract maps, huge moving mechanisms and complex puzzles - all kinds of extremely novel things that frankly don't get enough retrospective attention imo.
you can't call DN3D a clone unless you think that “first person game” or “game with guns” are ideas that wouldn't have been invented without id, which is patently absurd. both ideas existed and were being exploited from multiple angles, the self-suggesting combination of the two was going to have certain obvious and unavoidable properties, and the only reason doom seemed relevant to the conversation is because it was first. that in turn was incidental: 1993 was the point when computers became barely powerful enough to start making this kind of game, and doom did it first solely because id happened to have a programmer on staff who was talented enough to squeeze a 3D engine into the capabilities of the 486.
it's not like “texture mapped first person 3d” was a remarkable idea nobody had had before; everyone had had it in fact, since it was simply the natural progression of the nascent “VR” concept, which had been an object of obsession since the 70s. most devs simply didn't know how to execute on it, and since A) the PC meta in 1994 and 1995 was still overwhelmingly low-end 486s or worse, and B) doom was closed-source, its release changed nothing. i'm sure that lots of studios WANTED to clone doom, and there absolutely would have been a “flood of doom clones” if that had been possible, but even two years later it was still incredibly hard to make something doom-like that would run acceptably on the PCs most people had.
and thus, what mostly got released in doom's wake were not doom clones, but wolfenstein clones - massively technically inferior and not really featuring remotely similar gameplay, but far more achievable to devs who didn't know how to figure out what carmack had figured out. and even then, once again, the most successful ones (e.g. rise of the triad, blake stone) were literally licensed off the wolf3d engine itself, dulling the impact of the term “clone” and its implication of illegitimacy. so there was really virtually nothing that looked like doom - but by the time this was apparent, the industry had new business to move onto, and nobody bothered readdressing it.
the game that actually got cloned to hell and back was quake ii. by the time it came out computers were far faster and it was straightforward to make something on-par without needing to invent new science; you could simply plot polygons onto the screen in the most obvious way and it would work on most midrange-or-better computers. then 3D acceleration came along and shifted the challenge away from technical execution and onto design, e.g. making a game that was actually fun. this obviously left quake ii in the dust, and few people bothered commenting on it's genuine relevance to the development of the genre.
all this came to mind because i was watching jeff gerstmann play balloon fight, and he opened by asserting that it was a joust clone. i can't argue with this, it's definitely a joust clone… but who has ever said that before? how many “joust clones” exist? is it more than one? how many need to exist before we'd stop saying “clone” and just let it be a “genre”? and for that matter, do we apply this idea to any other artform?
in a sense, sure - movie reviews have always been willing to say something is “like a mix between [movie a] and [movie b],” and apparently Bush got accused of being “Nirvana clones,” but it's not really something that looms over a band or a film forever, unless they, you know, suck.
in the videogame field however, we're willing to point at a title and say, “this invented a new kind of gameplay, and now everything with similar mechanics will stand in its shadow, with an asterisk next to their names, unless they cross some unspecified and unknowable threshold. So Say We.”
nobody called Symphony of the Night a Mario Clone, but that is in large part just a matter of time, isn't it? it seems like there's no quantity of altered or superseded mechanics that quantifiably lifts the velvet rope and allows a game to pass into the hall of Original Works, and i feel like we just don't apply that level of conceptual exclusivity to much else. much to think about