With the sluggishness of the site these last few days you can sometimes get slowness or infinitely loading icon when hitting submit. If you aren’t sure if your post submitted, refresh the original page in a different tab before hitting submit again to see if it went through. I see double and triple posts in almost every comment section.
You should also know you can join smaller instances to help alleviate this problem. You’ll still see most of the same content, especially if the instance has active users. Don’t be too shy to sign up on some random instance, worse case scenario you just don’t get approved.
I joined my instance after being on Lemmy.ml about a month ago for a day, and there was so much lag because it was the largest instance at the time. I then switched to my current instance when it only had ~50 users on it and it made the world of a difference. I went from seeing the site crash every 5 seconds and posts taking 30 seconds to submit, to having a stable connection to the Fediverse.
Overall, I feel like everyone should try to join smaller instances to distribute the load so that you don’t have a bad experience, and the instances don’t get overwhelmed. It’s a win-win for everyone.