Some thoughts on that:
Duotight is just one of many similar products. These depend on the tubing being prepared well. If everything is as it should be, it won't leak.
Disconnect the line(s) - ideally only the one that's leaking! (find it using soapy water). They disconnect by pushing the ring on the connector inwards. Re-cut the end, making sure it is perfectly square. A cigar cutter might be useful, and they make similar items just for the tubing, but careful work with a sharp razor should do. Then plug it back in. IF it still leaks, the coupling is faulty and needs replacement, BUT one can possibly fix that with a bit of food-safe grease (vaseline, keg lube, etc.) smeared on the outside of the tubing - take it out, grease it, plug it back in.
Now if it is leaking somewhere else, like a threaded joint, add teflon tape or just throw your hands in the air and curse at it.