Lighter socket fusing

Our other vehicle is a Nissan Micra. The lighter socket works with accessories no problem and we have used chargers, sat-nav and dashcam. I put in a socket doubler I use in the jimny so we could use the sat-nav and dashcam together, plugged in one of them and it blew the car 15A fuse - but not the 10A fuse in the extension.
Went out and bought a new double extension and this one also blew the car fuse. Tried a 20A fuse then it blew the one in the extension lead. All of the individual things we plug in work fine but just plugging one of them into an extension blows the fuse.
These sockets are so simple I can't see why this would happen.
Any ideas most welcome!
Ray

What else is protected on the lighter fuse? It sounds like the fuse is nearly at capacity with the lighter and whatever else is on the circuit and adding the resistance of the extra doubler is tipping it over. I could be wrong and suggest you take it to someone who is competent to get it looked at.