The flag was originally designed by the anti-Qing group, Revive China Society, in 1895 with the addition of the red field component in 1906 by Sun Yat-sen in his speech.[4] This was first used in mainland China as the Navy flag in 1912,[5] and was made the official national flag of the Republic of China in 1928 by the Nationalist government. It was enshrined in the sixth article of the ROC constitution when it was promulgated in 1947. The flag is no longer used in mainland China due to the ROC defeat in the Chinese Civil War and the founding of the People's Republic of China in 1949. The ROC national flag has since then remained in use within the "Free Area", which includes the islands of Taiwan and Penghu, as well as some remaining offshore islands off the Fujian coast and some of the South China Sea Islands under ROC control.

I think maybe that's why, between 1928-1945 they had the same flag