What is causing people to accidentally double post?

Can't figure out what is causing this, this can occur dozens of times daily on media based subreddits. On rarer occasions, the same exact posts have been known to show up 4+ separate times too.

It's not the biggest deal having to remove duplicates every know and then. But when this flares up badly, especially during high traffic, it does get annoying.

Going to post this on r/bugs too.