What are the pros and cons of submitting same paper to more than one open repository?
At first glance, I don't think it would be an ethical problem, but it could be a management one: if you need to update the pre-print, you could take the risk to have inconsistent versions available. It could also be confusing for those who are looking for your paper: if I see a paper with the same title on two different open repositories, I might wonder if there is a difference between the two versions, download the two versions, realize they are the same, and dislike the fact that I just wasted some time.

Moreover, it could also be interpreted as an attempt to artificially increase your citation count, and that could be frowned upon. For instance, in Computer Science, DBLP is often considered as a good publication indicator. If you publish a paper on arXiv, on ePrint and at a conference later, that's 3 references for a single paper. Someone realizing this might think that you're just trying to abuse the system.