Might be true in case the filter has a large ring. When you mount it, the filter ring might act as a short lens hood and block some of the flare that was present without the filter. But this is the scenario I had to invent not to say directly what I think of this claim. [Bullshit.]
By the way, flare etc does not result only from UV, it results from all the light rays that reach the film plane but are not image forming rays. That is, if you have Milla Jovovich in your image frame, all the light that forms Milla on the image should originate directly from the one and only true Milla in front of your lens - NOT the light bulb above the jacuzzi or other light producing or reflecting objects.
Contrary to the common believe[?], tele lenses are more prone to flare than wides. This is due to the fact that in a small angle lens, from all the light reaching the front lens[without proper hood], only a very small fraction is the "good light", that is the image forming light. The rest are not supposed to reach the film/sensor; if they enter the lens and bounce around in the barrel and between the glass/air surfaces, they will certainly end up on the wrong place on the negative. This way dark areas can get light from neighbouring bright areas, or even from light sources that are out of the frame.
Ghosting is a specific form of flare; it needs a "good" DOUBLE reflection from 2 shiny surfaces such as uncoated or badly coated lens elements or filters. If e.g. the light from a candle hits a glass surface that is not or inadequately coated, it can be reflected backwards. Then it can meet a second glass surface and be reflected again backwards, parallel to the original rays from the candle. If the angle of incidence of the original rays on all the lens elements is not perfectly perpendicular (it never is in practice), the light after the second reflection will form an image on the film that is weaker in intensity but slightly shifted from the first-order image (the unreflected light), resulting in a "ghost" image. Depending on the coating on the lens elements, the ghost image can have various colours, or "natural" if there is no coating at all.
Sometimes an in-between situation arises, when the ghost image is a series of images of the diaphragma opening - but the effect is the same; the diaphragma blades are illuminated by a strong light source outside of the frame like the Sun, then the light that is reflected from the blades continues its way towards the film, gets double-reflected a few times on lens elements and this way it forms multiple images on the film. In this case the object is inside the lens so the multiple reflections forming its images pass through different lens elements, and the size of the ghost diaphragma images will vary (decrease or increase depending on the lens design).
