The Duet concertina explained

• Is fully chromatic.
• Has buttons which play the same note on each bellows direction, push/pull.
• THas low notes on the left hand side, and higher notes on right hand side. As with a piano, the melody is usually played with the right hand and the accompaniment with the left hand.
• Has an overlap in the notes present on the right and left sides hand sides.
• Can have 70 or more buttons and may be very large and heavy.  Some of the lowest notes on the left hand side are similar to the bass English system and are very big.
• Is often considered the hardest of the three concertina classifications to master. 

There are various keyboard layouts, or fingering systems. It is not easy to convert from one to the other:

Maccann Duet, probably most common.

Crane Duet (aka the Triumph) famed by the Salvation Army.

Jeffries Duet developed by Charles Jeffries, of Jeffries Anglo fame.

Hayden Duet, the most recent system developed (and some think the most logical).