There is some confusion out in the field over where double clamping is required in fuel system installations. The photo above illustrates the issue. In the photo you clearly see double hose clamps employed on a fuel line attached to the fuel filter. The potential problem with this installation is that the hose barb protruding from the filter housing may not have enough length to support the two clamps. Simply the second clamp out away from the housing may only be clamping on the hose either in part or the entire width of the clamp. If the hose barb is long enough, no harm done, but if it is too short, the more the clamp gets tightened, the more damage is being done to the inside of the hose as the end of the hose barb nipple digs into the hose liner.
So, the bottom line is that based on USCG regulations and ABYC fuel system standards H-24 and H-33, double clamping is only required on fuel fill pipes between the deck fill and the tank. Vent hoses are not required to use double clamps and fuel distribution hoses aren’t either. It doesn’t matter whether the boat is gasoline or diesel fueled.
Hope this settles the confusion!