Although there is no guideline or policy that explicitly prohibits it, writing redundant essays is discouraged. Essays should offer new perspectives or fresh insight on editing Wikipedia, and not reiterate arguments that have already been made in previous policies, guidelines, or other essays. Editors should use the many search tools that are available to avoid unintended repetition. Elaborating on a subtopic briefly covered in an existing essay, or offering a new perspective on an argument made by another essay, are acceptable.
Essays covering the same topic should be merged. If an essay is short and new enough that there is nothing to merge, then consider redirecting the page to a larger or more popular essay, userfying the essay if the author has a desire to keep it, or utilizing the deletion process.
Essay quality. It reduces overall essay quality. Multiple essays on the same topic spreads the work that could have been done on one page across several pages, leading to repetition and lack of depth. It's better to have one longer, high quality essay than many short essays.
Detracts from expansion. Writing redundant essays diverts time and resources away from improving on existing essays. Instead of writing a new essay, consider expanding an essay that covers a related topic.
New users. It confuses new editors. There are already enough policies and guidelines contributing to Wikipedia's steep learning curve. Hundreds of user-written essays, that may or may not represent community consensus, doesn't help matters.
Vandalism. Although vandalism in the project space is rarer than in the article space, it does occur. Having multiple essays on the same topic makes vandalism harder to detect, because there are more pages to watch.