@kevincooper it is possible that one of your plugins are causing the form to submit twice. Though tedious, try disabling each plugin one-by-one to determine which one is causing the issuse.
I have tested the classic editor plugin with the latest 5.1 and it seems to work fine. Maybe there is another plugin causing your posts to duplicate or perhaps the form is being submitted twice when you are saving or editing a post.
Are you using any plugins that add extra functionality to your post editor. Such plugins could be Advanced Custom Fields (ACF) etc?
Are you using the Gutenberg editor or the legacy WordPress editor?
