Not sure if this is the done thing here on this message board, but I have just joined up and I thought I'd say "hi".
Some of you may know me from melodeon.net or from Melodeons@Witney, and whilst the melodeon is my main 'folky' instrument, I have been playing concertinas in a mediocre sort of way for many years. In fact, I started off with a battered East German C/F anglo that someone gave when I was a teenager. The bellows were held together with gaffer tape, but it played and I learnt the basics which paved the way for my first Hohner melodeon.
But I do love concertinas very much indeed and I resolve to play them more. Not sure whether this is unusual but I play both English and Anglo quite happily and despite what people might say, I don't get confused between the two systems. The two systems are so different that it's like playing two instruments as dissimilar as, say, flute and fiddle.
I have come across people who strongly promote the English, being very dismissive of the Anglo, and of course vice versa. This seems very silly to me, as both instruments have quite different characteristics, their own strengths and weaknesses, advantages and disadvantages, and neither of them is 'better' than the other - just different.
And I have to confess I know next to nothing of the various Duet systems and probably at my time of life, I am content to let them remain a mystery and just sit back and admire the playing of those who have solved it.