Mizuta Madaisuki

The Mizuta Twins are two identical twins who were part of the Rumbar Pirates. They were identified by Oda in a SBS question. A fan asked why one of the Rumbar Pirates that died singing had a sword in his skull. Oda explained that this was because they were two identical pirates who died two separate ways.

Mizuta Madaisuki (ミズータ・マダイスキー), the older of two, died by the sword during the last battle of the Rumbar Pirates. His name is a pun as mizutama daisuki means "I love dots". He admired Brook's fencing style and wanted to learn it from him.

Mizuta Mawaritosuki (ミズータ・マワリトスキー), the younger of two, died as he was singing Binks' Sake for Laboon, before succumbing to poison. His name is also a pun as mizutama waritosuki means "Dots ain't bad".

They both wear polka-dot shirt with blue dots. The older brother has more dots on his shirt than the younger brother.

Anime and Manga Differences
In the manga, the skeletal remains of the older of the two brothers had a sword piercing his skull along with the another blade sticking out of his back. Also in the manga, his shirt is torn and his ribcage is exposed.

In the anime, his remains only have swords sticking out of his body, and his clothes remain intact enough to cover his ribcage. This was all done to censor the gruesome wound, which would be deemed inappropriate for younger audience.

Trivia

 * As shown above, the older of the two brothers seemed to have strong hair roots like Brook, for his hair remained even after turning into a skeleton.