Posting on multiple preprint servers can cause challenges both for authors and readers. Authors may find it difficult to keep all versions updated, and it can cause citations and other usage metrics to be split between multiple copies, making it harder to track downstream impacts of the work. Indexing services and search tools may also have to manage duplicate copies, potentially giving preference to the display of one over the other.
The impetus for submitting to multiple preprint servers is often to increase the potential exposure of a paper. This can be more easily accomplished by sharing the preprint with colleagues through email and social media.