For the most part, the characters on your list mostly started with DC. Though these characters aren’t necessarily the same, and if you think about it, there are only so many types of superpowers to go around. Like someone that runs really fast, is very likely to be in both. It also was more than these characters. Fawcett Comics originally owned Captain Marvel, there was a huge lawsuit between where DC took Fawcett to court over whether or not Captain Marvel (DC) and Superman were too much alike.
But for your list:
1. Solomon Grundy (DC) or Hulk (Marvel) – Soloman Grundy is from a 19th century nursey rhyme. But his DC debut was in 1944, Hulk never showed up until 1962.
2. Martian Manhunter (DC) or Vision (Marvel) – Martian Manhunter debuted in 1955. Original vision came out in 1940 via Timely Comics (Marvel’s predecessor) and the more commonly known Vision came onboard in 1968.
3. Doctor Fate (DC) or Doctor Strange (Marvel) – Dr Fate arrived in 1940 and Dr Strange in 1963.
4. Plastic Man/Elongated Man (DC) or Mister Fantastic (Marvel) – Plastic Man 1941, and Mr Fantastic in 1961.
5. Aquaman (DC) or Submariner (Marvel) - Aquaman came out in 1941 and Submariner 1939, making him one of Marvel’s oldest heroes.
7. Darkseid/Mongul (DC) or Thanos (Marvel) – Darkseid was 1970 and Thanos was 1973.
8. Metron (DC) or The Watcher (Marvel) – Metron 1971 and the Watcher was 1963.
9. The Flash (DC) or Quicksilver (Marvel) – The Flash 1940, and Quicksilver in 1964.
10. Atom (DC) or Ant-Man (Marvel) – Original Atom was 1940 and Ant-Man showed up in 1962.
11. Atomic Skull (DC) or Ghost Rider (Marvel) – Atomic Skull was 1976 and Ghost Rider in 1972.
12. Clay Face (DC) or Sand Man (Marvel) – Clayface was 1940 and Sandman was 1963.
There are lots more similar characters though, more crossover than you think!