@Anna999: I have had a look into this for you, and can understand your confusion. The best way to stay on top of this is to avoid using the Pictures tab in the gallery app, and move any photos to other albums via the default Camera album. For clarity, please follow the steps below.
Open the Gallery app > Navigate to the Albums tab > Select the 'Camera' album > Tap on an image > Tap the 3 dots in the bottom right > Move To Album > Select an album.
This will remove the photo from the Camera album, allowing you to keep on top of things a little easier. If you select 'Copy To Album' then the image will remain in the Camera album, and a duplicate will show in the Pictures tab. I hope this helps.
