When creating a new wikia, you may want to consider whether a wikia on that topic already exists. For example, if you want to create a wikia to document information about Star Wars, ask yourself if your wikia will be offering something that Wookieepedia does not. If your idea does not offer something new, we’d suggest that you not create a duplicate wikia but instead that you offer your expertise on the wiki communities that already exist.
Find a niche topic
Let’s stick with the example of a Star Wars wikia. Creating a community that is meant to serve as a comprehensive resource will likely not yield you any results. Wookieepedia has a years-long head start and already documents most information from the Star Wars franchise. Because of that, if you want to create a Star Wars wikia, you should find a specific topic within the Star Wars universe to create a wikia about instead of just an all-purpose Star Wars wikia.
There are already some prominent examples of wikias that have done this. The Star Wars: The Old Republic wikia documents information from the Star Wars: The Old Republic video game, while the LEGO® Star Wars wikia documents information from the LEGO® Star Wars franchise. Other topics have included Star Wars Fanon for fan fiction.
With that said, you want to figure out if you can create and nourish a sub-topic of the overall topic. Competing with or overtaking a wikia that has been established for years is highly unlikely, but further developing a sub-topic in its own wikia is certainly possible. Besides, one of the core philosophies of wikia building is collaboration, not competition. You should find a way to co-exist, and a niche topic can do just that.
Create and promote uniqueness
So, you’ve created a niche wikia. Let’s say your wikia is called the Jedi Wikia, and your goal is to provide lots of great information all about Jedi. There are lots of things you can do with a niche topic like that, but you need to promote the fact that your topic is offering something that the main wikia does not. To do that, you need to offer something unique.
If the end result of your effort to create a niche wikia is to cover the exact same information as the main wikia, then your wikia has proven to be an unnecessary duplicate. You want to offer something that the main wikia does not. For example, Wookieepedia covers in-universe information (biographies of characters, timelines of events, and so forth), but the Star Wars: The Old Republic community covers gameplay information for the video game. That allows The Old Republic wikia to be a resource for gamers in a way that Wookieepedia does not, and it is what makes The Old Republic community successful. That is the type of niche topic you want to find and cultivate.
Now that you’ve found that niche topic, you want to promote that. Make your main page say things that are clear in how your wikia differs from the main one. You don’t need to actually list differences in comparison, but there could be key buzzwords that provide a reader with the means to understand that difference. For example, a video game wikia like The Old Republic Wikia could say something akin to “a comprehensive resource for gamers,” which shows that it would specifically target a gamer as opposed to someone looking for historical information on Star Wars lore.
In the same way that you can attract contributors to your site, consider how your wikia is promoted in general. If you have a Facebook or Twitter page, for example, promote it in a way that showcases its uniqueness. If you are promoting a wikia like The Old Republic, promote it in a way that targets gamers. That way, your core audience can know what your topic is and gravitate to it.
Start a friendship with the main wikia
One of the best ways to help promote your topic is to be on good and friendly terms with the main wikia. Take the Star Wars wikias as an example. Wookieepedia lists LEGO® Star Wars, Star Wars: The Old Republic, and other Star Wars wikias and websites as their official friends. That gives those wikias a noticeable place on Wookieepedia’s main page, and it acknowledges the fact that these wikias share an overall topic but are offering their readers and contributors different things. Wookieepedia has chosen to be a resource that documents the in-universe lore of the franchise, so they can direct gamers and gameplay enthusiasts to the Star Wars: The Old Republic community. That makes that friendship a sort of partnership that benefits them both and respects both topics.
Even if there is no official friend process like there is with Wookieepedia, having a relationship and an acknowledged and respected difference in the focus of the topic is a good thing. It allows both wikias to co-exist under the umbrella of one topic while respectfully working towards shared yet slightly different goals.
Of course, that friendship may not always happen. There may be a main wikia that wants to be everything and own everything about a topic, and they may not like the fact that another wikia is competing with that. For your part, don’t worry about that. Even if your wikia is a duplicate, there’s nothing inherently wrong with that. We do allow them, after all.
Should you find yourself in a dispute, the best thing to do is to ignore it unless it becomes a disruption on your wikia, in which case you have administrative tools to help with that. You can also contact Wikia for help and advice in that regard. Remember that you are free to create a wikia with a similar topic to one that already exists, just try to make it somewhat different as advised earlier in the page.
Now, the following advice is for those who are disputing the new wikia’s existence: unless the new wikia is directly copying content from your wikia without attributing it under the terms of the Creative Commons copyright license, there is no need for you to make an issue out of its existence. If you find yourself annoyed or upset that it was created, the best thing to do is let it go. You are already on an established and successful wikia, so you really have nothing to worry about.
Instead of debating the issue, try to become friends as outlined earlier. It really is the best thing for everyone!