EDIT: I am now editing this post, don't think it will make a double post.
This can happen in a few situations:
1. You click the submit button once and then press again thinking it wasn't properly pressed earlier. This could result in 2 submissions.
2. When you are typing a new post, if you click the Submit button and then go back to the previous page using the Back button of the browser and then submit again, it will become a second submission.
Please take care of the following:
1. Press once and then patiently wait for it to get submitted, especially if you have slow connection.
2. When you are submitting a new post, do not press the back button on the browser to reedit. Instead, use the edit link within the post to reedit the post.