I uploaded my original abstract background and texture designs to my contributor account, but many of them were rejected for 'Similar content in our collection'.

​@mishukhagidok,

In addition to what’s available on free websites, there are over half-a-billion similar assets in Adobe Stock. See related topic below.

Oversaturation is the most likely reason for refusal. Stock is a business, not an abstract art gallery. 

Offer customers what they want and you’ll succeed at Stock. For inspiration, read through your Contributor User Guide. 

Popular Subjects:

https://helpx.adobe.com/stock/contributor/submit-your-content/submit-photos/photo-submission-overview.html

Hope that helps. 

There are quite likely many other backgrounds similar to yours in the database. Try using search terms from your submissions and see how many similars show up. They are easy to create, and for quite some time now, at least 1.5 years, Adobe has been almost automatically rejecting them. You’ll need to look for more unique themes for your future submissions in order for them to be accepted.

A search for “abstract backgrounds” yields over 110 million results. While it’s generally felt that similar content rejections can sometimes seem illogical or over-aggressive, this is one case where such rejections are probably reasonable. A search for “textures?” Nearly 150 million results.

Rejection due to content similarity applies not only to the assets themselves but also to titles and keywords.
If a large number of assets with similar titles or keywords are already listed on Adobe Stock, simply using the same titles or keywords for your own submissions won't lead to sales unless they rank high in search results.
The thematic approach you take when creating your work is also crucial.