There are a numerious things that can cause 'ghost' images.
Assuming you are not referring to normal IR issues like thermal reflects off low emissivity surfaces and instead, the next question would be, what do you mean by 'ghost' images. Are the patterns "burned" into the image or are the blurring image trails, that appear when you move the camera.
If you are seeing the latter, the P640 has an a Noise Reduction Mode which uses image averaging. If it is turned on to the maximum noise reduction mode, it cause pronounced blurring/smearing of images when the camera is moved.
If images seem to get "burned" into the detector, I would contact the service department. Stuck or malfunctioning NUC paddles on any Microbolometer based camera can causes a images to be NUC'd into the offset/correction making a it appear that the image is burned or ghosted into the the detector. A problem like this is typically very easy to correct, and only takes a minor adjustment by the service department.
While I have not heard of this problem on the P640, I seen it occur on other cameras. In this case, you may want to make note of the ambient temperatures when this occurs. I've heard of cases where the camera works fine a room temperature, but the NUC paddle sticks when in higher temp enviroments. I've also seen cases where the paddle sticks when the camera is first turned on or after it has been stored in cold enviroment, but the problem goes away after the camera warms up.