Sporadic Multiple Form Submissions?

We've been dealing with this issue for years and it occurred to me that I should ask here to see if anyone has experienced the same problem.

We use HTML landing pages and pages created with the builder. On our HTML landing pages we often (but not always) get multiple form submissions from one user that are just 1-3 seconds apart from each other. These are legit form submissions from actual clients; not spam or bots. It doesn't seem to be a user issue because we've tried replicating it by submitting the form quickly and we aren't able to do it that fast.