There's a strange ghosting effect in this photo (actually quite a few of my recent photos), when I used built-in camera flash.
EXIF: f/5.6, ISO 400 and 1/60 sec.
Is it caused just by a slow shutter speed? How can I remove it?
For me this is motion blur. And because is the same on all the objects edges it is caused by slow shutter speed. Try to use 1/100, 1/160.
Also you should know internal flash usually have very limited power and range. So the other advice I can give you (if you often take photos in low light) is to invest in external flashlight.
To be a bit more specific, this is the superposition of
However, the amount of motion blur for a 1/60 shot is unusually large, so you likely took the picture from far away. This would also explain why the blurred part is so visible, your flash was too far from the subject and didn't make much difference with the ambient light.