It is possible that some app you are using is making copies of your photos.

A more common possibility is that there is a '.thumbnails' folder that is full of low resolution copies of your pictures.

Use your file manager to locate these folders, and then use the 'show hidden files' function.

Look for this .thumbnails (the dot in front is important).
Many crappy (ie. stock) gallery and video apps require these little pictures of your pictures in order for you to see the little versions when you search through your media.

They are not really needed, take up space, and cause confusion.

A great gallery that does not require them at all?

Simple Gallery Pro
https://f-droid.org/en/packages/com.simplemobiletools.gallery.pro/

To eliminate the .thumbnails folder and keep it from recurring, you must first delete it and then replace it with an empty file called .thumbnails.

It is imperitive that you remember to delete the FOLDER and replace it with a FILE.

Android will not allow two items with the same name to exist in the same location.
If you delete the folder, then create another folder with the same name, then the folder will just do what it did before.

So replace the folder with a file.
That way, when the pesky app tries to make a .thumbnail folder there is already a file there with that name.
So it cannot make the folder.

Also, keep in mind that apps like to save pictures and videos wherever THEY want, which is often not where we think they should be.

Apps make their own folders, sometimes with names that seem to have nothing to do with whatever app is using them.
(So frustrating!)