I have not had my story done first, but I have encountered some suspicious coincidences.
Back in college, I told a friend about an idea I had for a one-shot comic, but did not have time to complete. Sure enough, two months later, he turned out a comic with the same idea. It is likely he stole the idea, but also just as likely that he forgot the conversation and remembered the idea. Either way, I never vocalized my ideas again without laying the foundations for it myself first.
Next, I had a humble webcomic that went on for about 5 years in the early 2000s. As I was getting burnt out between my full-time job and trying to maintain my art, I noticed a Tokyopop manga had come out with a MC with a similar name and character design. Similar enough to raise an eyebrow among people who knew my work, but not similar enough to be theft.
TLDR: Cover your butt.