23rd Oct: It's time for the second TV Tropes Halloween Avatar Contest, theme: cute monsters! Details and voting here.
The below is a list of shows that are considered twisted reflections of each other. In some cases, one is the original and the other is a rip-off or a "me too" show, but which is which is not always completely clear. Other times, multiple artists produce something based on trends they see around them, and the end products happen to be similar. This phenomenon is known in the wild as convergent evolution. Often, the shows are developed at the same time, so all they have in common is their basic premise and pitch description. This is especially true in animation, given the long lead time required to get a finished show to air. When both programs get to the screen, they may differ in every important respect, except their one-sentence capsule description. Each program has its own audience and meddling suits. Either way, if they persist in coexisting, the audience will polarize, and the shows will evolve away from each other, or one will be canceled. Sometimes, rather than home-brew or a knock off, a company will license a foreign program, usually Anime, and adapt it to be more like its competitor. Rarely, two dissimilar shows will develop a rivalry. Maybe they're opposite each other in the same time slot, maybe one steals the other's time slot, or maybe there's some superficial similarity that causes viewers to compare the two shows. Arrested Development had a whole episode full of in-jokes about how people compared it to The O.C., simply because they were both on Fox and set in Orange County, California. Reality shows are more likely to develop actual rivalries, are more likely to appear as blatant ripoffs, and are more frequently appearing on the below lists in general. Various factors — the high ratings and inherent low production values reality shows bring to the table being chief among them — tend to mean that both cable and broadcast networks love playing follow the reality show leader. One thing to remember regarding Dueling Shows is that Tropes Are Not Bad — while many dueling shows appear to be blatant rip-offs, the competition between two shows (or simply the desire to set each other apart) often spurs each other to develop in quality. The result is a grateful television audience. See Dueling Movies for the cinematic version and Dueling Games for the video game version. Do not confuse with Fighting Series. Again, just to be clear: The shows have to air during the same time period, otherwise you've got either Follow the Leader or Serial Numbers Filed Off. Also see Fandom Rivalry.