There actually is a standardization, Double Flare are always Standard dimension, and Bubble Flare are always Metric. The fittings are noticeably different where they seat on their respective flare, and as mentioned, the threads are different as well.
That said, I have come across a hybrid flare, that is a combination of both a double and bubble flare. I've seen them mostly on FWD GM products, but have also seen them of Ford trucks.
