How to find out if your cover art has been used elsewhere
It's bound to happen whether you're an indie or published with the Big 5. Occasionally, you'll find your cover image reused--sometimes straight out of box on someone else's book.
It's irritating, but it happens. Popular images tend to get overused.
If you'd like to see if your cover art has been used elsewhere (and how). Right click your cover and click on View Image Info. The url of that image will already be selected. Copy and paste into Google Images.
If Google has found that image anywhere, it will spit out all the places it's seen it. It will even list any images that look similar to it.
By the way, this is how photographers and artists find out if someone has used their images without permission or payment.
It's also a reminder that you are simply buying the license for the art, not the art itself.
There's not much you can do but either customize an existing image, or have a cover designed and buy the rights to it.
Does it bother you to see the same image used on different books? I think it dilutes the message a little. And I can't help but feel a little sorry for the authors. Everybody wants a unique cover.