Haven't I seen that somewhere before?

Let’s say you are an author happily perusing the bookshelves at Apple, Barnes and Noble, or Kobo looking for new authors and interesting titles. Let’s say you happen to see a book that looks almost like yours. Perhaps it has the same title. I mean exactly the same title. Heck! It looks a lot like your book.
 
It happens.
 
There’s no way you can ever know if the author was lazy and didn’t bother to do an online search to check the title did not already exist before they published. Or perhaps yours caught their eye and somehow they forgot they saw it and woke up one morning with a brilliant idea for their title.
 
It happens.
 
Perhaps they liked yours so much they just had to have it for themselves, hoping to gain momentum off the back of your success. Well, if it was this last I suggest you take it as a complement.  

Really, the old saying there's nothing new under the Sun remains—for the most part—a truism. We all take from others whether we know it or not. Some are better at acknowledging who’s shoulders they stand on. 

Let’s say an author takes not your title but the image on your cover, if it is awesome enough, it is going to be borrowed by others. Case in point, my book. Jubilee Year was published in 2015 with a cover image that juxtaposed a (burning) observatory with a meteor (shower). Now, funnily enough, you can find exactly that image, be it simplified, on at least one book published this year. This book actually shows up on the first page of the book carousel adverts on my Jubilee Year book page. The book carousel of adverts is the second line of covers, the one under the ‘Also Bought’. Now that's pretty darn cheeky!

If I wanted to indulge my fantasies about being a trend setter, I could argue it sparked a whole new trend in book covers featuring meteors over the landscape of planet Earth. But of course, there were plenty of books with meteors and comets and asteroids and stuff on the covers before my book came along.

I know that my title May Day is in fact used on multiple books on Amazon. But honestly, when I first came up with it a couple of years back, I could only find a couple of old books with the title. I wrote to that title so I really did not want to change it by the time it came to publishing time. When it's combined with the series title it doesn't matter so much because it is clearly defined apart from the other books sharing the words 'May Day'.

That was a bit of a ramble, wasn’t it?
 
In short, don't worry about seeing others borrow your ideas (stealing your entire book is a whole different ball game). The best solution is to write another good book. That way when potential readers think of that title, or that cover, it's your book they have in mind.