Fittings with redundant seals for aircraft fuel lines, fuel tanks, and other systems are disclosed. A fitting in accordance with one embodiment can be positioned adjacent to an aperture in a fuel tank wall having an inner surface and an outer surface. The fitting can include a first member having a flange and a barrel. The barrel can further include a passageway. A first flange seal can be positioned at a face of the flange and can be sealably coupled to a surface, for example, the tank wall. A second flange seal can be disposed outwardly from the first flange seal against the surface. The fitting can further include a second member (e.g., a sensor) positioned within the passageway of the barrel. The fitting can further include first and second barrel seals positioned between the barrel and the second member.