On my Duetto (2002) the shower mixer and the kitchen sink tap are virtually identical. The shower tap started to drip. Assuming your on the same taps then the tap can be fully striped from the top. Remove the small blue/red cap on side of control which gives access to an allen bolt, slacken off and handle can be lifted off. Rest of tap works can be unbolted, watch carefully where all . seals go. I cleaned all internals and put back with a small amount of silicone grease and leak stopped. So assume it was a piece of grit or limescale that caused problem.
Jim

 

Duetto water tap

[ Jim . Thanks for your information. The one I have is in a 1994 Duetto and I had previously tried to remove the small red/blue caps ( about 3mm dia.)without success and tried to get some descaller down the tap. Will now try a little harder to see if there is a screw under it.