If the old line leaked. And the new(old) line leaked in the same place in the same way, it's not the line or it's nut. It's the fitting you are screwing it to.
If you cross threaded it the first time, if you pushed that too far, you may have damaged the threads on the IP fitting.
Alternatively...
Obviously, it was leaking. Or you wouldn't have tried to fix it.
So, how long was it leaking?
Had it just started leaking?
Again, I would be looking at the fitting. But also the possibility that something further to that is cracked or otherwise leaking.
Again, if both lines leaked, I would bet that if you get a brand new one, and get it on there just right, and guttentite, it will still leak. Because the problem is the fitting itself or further in.