Lightroom is exporting ALL of my files at once.

Lr-Classic: Thanks!

I think you may be right about Ctrl+A, I may have inadvertantly hit that combination, believing I was doing a different function.

[Ctrl+A] can be very annoying if you frequently view you photos by clicking on the "All Photographs" list. [Ctrl+A] would then select EVERY photo in your catalog. (thousands!!!). View the Grid with a folder selected- [Ctrl+A] will select all the photos stored in that particular folder (and possibly any sub-folder images viewed in the grid).

[Ctrl+D] is a good shortcut to learn as it De-selects all images viewed in the grid. Then you can be more certain that only the images you now actively select are the photos you will export.

Where do I check the "Export dialog," and which setting should I be using?

That's an open question requiring much explaining. "which setting" depends on the 'Why' you are exporting!

The free eBooks are a great source of info to help with these basic topics.

https://www.lightroomqueen.com/

You do know that you do not have to export photos to 'Save' them, right?

https://www.youtube.com/watch?v=jAkEWfpxH7U

Also, would Ctrl+A continue after I've closed and reopened Lightroom?

TBMK (never had to try !) If you have selected 'all' photographs with [Ctrl+A] then, maybe, it would continue exporting ALL the selected photos until you cancel the export by clicking on the [x]

So, your Lightroom is - Lightroom (CLoud based), Lightroom-Classic or earlier version (local folder based)?

Sounds like you had ALL photos selected by [All Photographs] and [Ctrl+A].

In (local folder versions) You can interrupt (cancel) an export by clicking on the [x] on the progress bar.

"In duplicate" - may mean you repeated the Export.

What are your settings in the Export dialog?