I feel like the experience should make you extremely cautious of using stupid tools to analyze your codebase, not the introduction of external code.
And it seems the software itself reflects a misunderstanding about how programming code is actually manifested. It's very possible to read the official docs or use an IDE and end up writing the same lines of code in some Stack Overflow answer, as many here have said already.
As you say, there might be a case where two independent implementations came up with the exact same code, although the probability of this is reduced exponentially with the size of the exact match. Keep in mind, we do not talk about "i = 0;" here, but something longer. And we do not discuss examples matching the official documentation, because then you can point to it as origin, instead of SO -- and it would probably be under a better license.
The questions are: even if you wrote it yourself, do you justify the costs of potentially going to court to defend the case and are you sure you will convince a judge, who will be the person making the decision?