Advertising should not cannibalize existing products. It is always better to have different segmentation targeting positioning (STP) thought out for different products in advance so that they do not eat into each other's share of customers and leading to a situation called parasitic advertising.
Parasitic advertising can also be across different firms. It leads to direct comparison of products, and may turn nasty leading to law suits.
Example: examples of parasitic advertising can be the campaign of Dove causing a reduction in sales of Lux, both of which cater to the same segment of consumers.