I wanted to say,
Driving a car is like a second nature to me.
But Google Docs and ChatGPT both corrected my sentence to:
Driving a car is like second nature to me.
So which is correct, and why do we remove the "a", but use the "a" if
Detecting a scam is like a natural instinct to me.
What exactly is "a second nature" or "a first nature?" It's more just an expression people use. The expression is "second nature" with no preceding article, though it isn't that wrong to include the article. However, there is no "like." If you add "like" then it sounds like you need "a" which it seems like you're picking up on. But
Driving a car is like [a] second nature to me.
Driving a car is second nature to me.
It is similar to:
Detecting a scam is natural instinct to me.
(Note: natural instinct isn't an expression, so I wouldn't use the above sentence.)