Fittings with redundant seals for aircraft fuel lines, fuel tanks, and other systems. A representative fitting includes a first member having a flange and a barrel. The barrel further includes a passageway. A first flange seal is positioned at a face of the flange and can be sealably coupled to a surface, for example, a tank wall. A second flange seal is disposed outwardly from the first flange seal to seal against the surface. The fitting further includes a second member positioned within the passageway of the barrel. The fitting further includes first and second barrel seals positioned between the barrel and the second member.