This has been happening on two different devices, but using the same flakey wi-fi with both. I've seen my submissions be duplicated on SN for Posts and for comment replies as well.

Like right now, when I press the reply, it might toggle to disabled while it submits, and that might stay that way for like 10 seconds or maybe longer. SN is not the only site / service where this type of extended pause for a response will happen, but I've not seen the a duplicate submit occurs with other sites I might use. I can confirm my wifi frequently has latency issues, including times when I'll see like up to 6 second response times for ping (!), for extended durations -- 30 seconds or 45 seconds maybe, then things just resume at normal speed. Whether that's due to broadband connectivity, or maybe RF interference, or whatever, I have no idea -- and it's not my access point so I can't do much further troubleshooting. But I know I'm not alone in dealing with a flaky wi-fi connection.

There was a fix to Issue #94 in SN's github repo, but that doesn't seem to have been implement for comment replies (as same rule could apply, no duplicate comment reply, from the same user at the same comment level). And I am pretty sure that still is occurring for the SN Posts too, so maybe that fix got reverted, or was ineffective?