There are 2 sources of "similars" rejections - those within your own account and those across the entire database. Submitting too many similars in your own account is forbidden by the guidelines (read them again if you're uncertain), and can result in account suspension. Similars across the entire database means that Adobe just has too many of that type of image already and doesn't want to flood Buyers with even more. Backgrounds are definitely in that category. Searching for the keyword "background" nets more than 31 million images. You can search the database yourself to see how many backgrounds exist to the ones you're considering uploading. It is believed that such rejections don't harm your account in any way, but they do waste a lot of your time.
None of us knows how Adobe determines if submissions are similar to others in the database. Some think that it largely relies on keywords. I submit only real photographs of real places, and I know for a fact that some of my "similars" rejections are unwarranted, as they are truly unique in the database. Unfortunately, there's not much we can do about it. Sometimes I re-edit an image and alter the keywords a bit and resubmit. If it's not accepted in the second attempt, I just move on.
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