Had a duo for 18 months, made that horrible noise from about 13 months (just outside of warranty as usual).
Bought an envirodri, what a difference!
Sebo duo - Good points - Lightweight, okay for most domestic jobs, easy to service, cheap!
Sebo duo - Bad points - Too lightweight for some domestic and most commercial jobs, dodgy switch, bit flimsy, brushes expensive and wear out quickly!
Envirodri - Good Points - Excellent for agitation and dry compund cleaning, different brushes for different tasks, heavier for better penetration, better built!
Envirodri - Bad points - Heavy (especially getting up stairs), 4 x more expensive (but you get what you pay for), brushes more expensive (but last a lot longer).
Neither machines are made for agitating pre-sprays or being in a wet environment so using for that purpose will invalidate any warranty. however, I (and most others) use them both for agitating pre-sprays and both serve a purpose.
If you're on a limited budget get a duo, good little machine.
If you've got a slightly bigger budget go for an envirodri or host, better machines.
Kind regards,
Jason.