Setting backgroud image results in duplicate style rules

Hi @artf, I have one strange little problem. I have a custom trait which has an implementation for setting images and background images. When I set the background image a second time by using the code below I'm getting duplicated styling rules on the element Id. I already played around a bit with the forceClass and avoidInlineStyle but it doesn't really help with this problem. Maybe related to this problem: The element also has a class with a default background image. When I open the StyleManager I only see that background image in there. Not the images from the Id style rules. Looks like it...