Well, I came across some duplicate wikis on Baka to Test. While there's no any rule about creating duplicate wikis generally on Wikia, but starting one from scratch really hard. It takes a lot to work the way up, harder still if only one person doing it. We're not complete here either, but at least we already have some image database to use.
Other thing, the creation of other duplicate wiki, is because of the strict regulation here. We do allow comments here, but no using of cursing/sexist etc...just be polite. Yes, the series does have pervert moments, but that doesn't mean we allow obscenity or profanity in this wiki.
Mostly, every misunderstanding can be resolved by having some conversation. The duplicate wiki I saw, I believe the user/founder really misunderstood here, and instead of facing it, (s)he went and create another wiki.
Seriously, if any of you are not satisfied, just say it out here, I'm not gonna eat you. I'm not crazy about blocking others either (just maybe for the obvious spammers/vandals), that's what special about wiki, building a community by discussing everything. If no one giving feedback to the admins, we won't know your opinion(s), and finally it will just seems to be admins having the final say to everything. We're not dictators here.