The Flash’s kids have the same name if you have the Don-Dawn merger.
Over on the tumblr The Chronological Superman, “Calamity” Jon Morris says
Some genuinely weird stories appear in the catalog, such as “Superman’s Lost Century,” the epic Mordru arc beginning in Adventure Comics vol.1 No.369, and the debut of Barry Allen’s immediate offspring Don and Dawn Allen a.k.a. The Tornado Twins in Adventure Comics vol 1 No.373 (Oct 1968). Who in the world would give their twins a homophone for a pair of names? I mean, the answer is Barry Allen and his wife Iris, but I pose the question rhetorically.
He’s right – the Mordru arc is a genuinely weird Superman story. No, I kid. He’s also right that the names Don and Dawn are pronounced the same by many speakers. It’s called the low back merger and it’s also what makes people pronounce the words cot and caught the same. Without the merger, the word Don is pronounced /dɑn/, while the word dawn is pronounced /dɔn/. So it’s only the vowel that distinguishes them, with the first vowel being more open and farther back in the mouth than the second vowel. But in the merger, the vowel in the word dawn shifts down to the vowel in Don and they become homophones.
So who has this merger? Well, according to the Atlas of North American English, the merger is “is characteristic of a very large part of the geographic terrain of North America” (Lobov, Ash & Boberg 2005: 60). The Atlas gives this map, where people who are inside the green line have the merger and the green dots represent people who both hear and speak the words Don and dawn identically.
It makes sense that the names of the Flash’s kids could be Dawn and Don. Barry Allen, aka the Flash, is from Iowa, which falls outside of the merger boundary in the image above. And he operates as the Flash in Central City, Missouri, another place outside of the low back merger area. The only thing is that the Tornado Twins Don and Dawn were born in the 30th century, which proves that the low back merger will never fully sweep across North America. Even 10 centuries from now there are places where Don and dawn are pronounced differently. Now you know.