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Im facing the same issue. I uploades 2 png images with 2 sets of overly which looks completely different from each other. One gets accepted in hours and the other one get refused for this reason after 4 or 5 days. I think my problem is using the same keyword as the guidline pages says, if your keywords and titles are same that can get it rejected. I also noticed unlike most other platform like this, contributor here tends to upload 1 file with very large set of something. Where in other sites you see many files of same catagories with smaller collection in a single file. Now i know why. 

Honestly, you really can't. There is no mean that assures you that your pictures won't get struck with this refusal. My guess is that the tool they use is not working correctly or is ill configured. They pretend their tool works as intended.

This does not mean that you should submit knowingly similar assets either in the submission (e.g. 100 pictures of the same dog) or in your database (100 pictures of the same dog over a longer period in different small batches), as this can get you really in trouble. Accounts like that may get suspended if caught. You also shouldn't resubmit persistently the same assets, as this can get you banned too. (Some contributors will advise you to do exactly this, despite the clear instructions on the Adobe web site not to do that.)

To date, I've had 150 submissions rejected for being "too similar." Some perhaps rightfully so, others that are at least arguably unique. I've also had assets accepted that I felt certain would be rejected as too similar (wallpapers and backgrounds, for example). So there are really no guidelines that I personally can recommend, other than to pay extra attention to your titles and keywords. I don't use keywords or titles that do not represent the actual content of my submissions, but I will try to think outside the box while leaning toward being very specific about the content, rather than generalizing. For example, I do a lot of portrait and fashion images, and Adobe's suggestions for keywords always include words like fashion, beauty, glamour, etc. I either avoid such words or place them at the very end of the list. Sometimes, but not always, those images will bypass the gatekeeper and be accepted.

You can search in the Buyer portal stock.adobe.com for images with a few of the most important keywords you are using in your images. Some categories are very competitive, and Adobe doesn't any more images similar to the many that are already in the database. Nevertheless, many have complained that even if their images aren't very similar to other existing images, they are rejected anyway. Such rejections don't place your account in jeopardy. However, you must not submit a bunch of similars in your own batches. Doing so can be considered spamming and CAN get your account suspended.