Eijiro and Tetsutetsu Tetsutetsu share many similarities with each other:
They share the same birthday (October 16).
Both have prominently pointed teeth.
They share the same stats in Ultra Archive Book, prior to the release of the Ultra Analysis Book.
Them and their Quirks were formally introduced in quick succession, while in identical situations: emerging from the wreckage of Villain Bots.
Both wanted to stand out during the U.A. Sports Festival, but were disappointed that they couldn’t, upon realizing their similarities to each other.
In the anime, their U.A. Sports Festival entry poses were the exact same.
Their Quirks, Steel and Hardening, are functionally similar and described likewise, including the weakness of losing power in a drawn out battle, and making their bodies harder than stone.
Their fighting styles are the same: straightforward and reckless, letting their hardened bodies take opposing hits while delivering counterattacks.
Both use their Quirks to turn themselves into human shields to protect their allies.
They both chose Fourth Kind's office for workplace experience.
Both have similar hero costumes.
Both have a Super Move that involves using their hardened fists to break through the opponent's defenses, while having the same pose when performing their Super Move.
The beginning of their Hero names start with the letter "R".