Themes have the ability to download jQuery. It is one thing that some themes use their own version of jQuery (to avoid problems when WordPress updates and updates versions of jQuery). Usually the admin section in jQuery always includes it.
If you can, try writing cross-version jQuery (if you want many sites to use your plugin).
To answer your question directly, it is best to add your own enqueue_script jquery ... it doesn't hurt to have multiple calls for enqueue jquery.
Adam erstelle
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