What letter of the alphabet has been used as a mintmark for two different United States mints?

The letter D has been used by the Denver, Colorado mint since 1906. There was also a mint in Dahlonega, Georgia, opened in 1838 but seized in 1861 by Confederate Army forces and closed.

The letter C can also be considered to be used twice if you consider that it was used by the Charlotte, North Carolina mint from 1838 until 1861 when it too was seized by Confederate forces and closed. The letter was also used as CC to indicate coins minted in the Carson City, Nevada mint from 1870 to 1893. That press is still there - and still works.

https://www.littletoncoin.com/webapp/wcs/stores/servlet/Display%7C10001%7C29555%7C-1%7C%7CLearnNav%7CMint-Mark-Guide.html?gclid=EAIaIQobChMIn7S4juj4-AIVUB-tBh3oxgQ2EAAYASAAEgKaoPD_BwE

The "D" was used by the Dahlonega Mint located in Dahlonega, Georgia. The mint was established by the Mint Act of 1835 for the coinage of gold. The mint produced coins every year from 1838 through 1861. Denominations included $1.00, $2.50 (quarter eagles), $3.00 and $5.00 (half eagles).

The mint mark "D" was later used by the Denver Mint which started on February 1, 1906. The Denver Mint is still operating and producing coins for circulation as well as commemorative coins. It is today the single largest producer of coins in the world

https://en.wikipedia.org/wiki/Dahlonega_Mint
https://en.wikipedia.org/wiki/Denver_Mint