(Note, I am not affiliated with this plugin, I’m just another user.)
Good thinking to backup your site before experimenting. But Tools > export and Tools > import are not really intended as a way to backup your WordPress. They are mainly for when you want to copy all of the “content” (including posts, pages, comments, custom fields, terms, navigation menus, and custom posts) from one WordPress site or installation to another one.
There are good plugins for backing up all of your plugins and restoring them after trying some new things. And there are even more plugins for backing up absolutely everything about your site and restoring it to a given state. But the built-in tools for exporting and importing your site “content” don’t qualify for either of those purposes.
Mechanically speaking, what happened when you did that: when you exported, you made a serialized (as XML) copy of all of the content mentioned above; then when you imported, you installed that copy of all of those things right back into your same site. The system doesn’t expect you to do that, so it has no way of knowing or checking that maybe you were importing a duplicate of every item. So it just did its duty and created a new copy of every single one of those things that you imported. That includes all posts, all pages, everything. And the one that was visually most noticeable first of all, it created a new copy of your menus.
Unfortunately there is no “undo action” for this, no simple one-button fix. But you may be able to clean up fairly easily by the following: 1. copy your exported file safely to a location outside of your site; 2. install a WordPress Reset plugin (there are many, and most of them work okay, so use the one that has the most users, the best reviews, and is compatible with your WP version) to reset your site; then 3. do Tools > Import and import your content file to get your content back. But I cannot promise this will all go perfectly. It will for most sites but it’s just an idea and I cannot guarantee it. The WordPress Codex and other online documentation have lots of good pages on how to fully back up your site’s database and files before doing anything risky. I wish you smooth sailing and a good outcome.
