Should you continue with plans to write a novel if you find a similar one that’s been written before?
Of course you should!
A publisher — can’t remember which one except that it was in New York City — said that one of his biggest problems was writers who contacted them threatening to sue because they’d submitted an article that was rejected and then, later, the magazine printed a similar article. The point of the publisher’s comment was that it’s common in the publishing world to receive numerous similar submissions. In many cases, some of those rejected submissions were later published elsewhere.
But going deeper into the question of similar plots, there’s a list of story plots that’s easy to find online (because there are a lot of them and all are slightly different) because there are only so many story plots.
The thing that differentiates one story from another with a similar plot is the way the story is written. Since everyone is different — and every writer differs from every other writer — each story, even those with similar plots — will be unique.
So go for it!