Scenario

1) enable 'write metadata to files' in the preferences
2) import photo "picture-of-car.jpg"
3) add tag to photo "picture-of-car.jpg" <car>,<blue>
4) import photo "picture-of-car.jpg" again

the photo "picture-of-car.jpg" will be saved as "picture-of-car-1.jpg" in the Showell library folder structure leaving two of the same photo with different tags in the library. "picture-of-car.jpg" with tags <car>,<blue> and one without tags "picture-of-car.jpg".

The expected behaviour would be that Shotwell does recognise the image by a fingerprint of the actual image (without metadata) and would then offer to merge/replace/add the metadate of the new imported file with the existing file.