Insignia, meaning "a distinguishing mark, sign, or badge," is both singular and plural in number and has an additional plural form, insignias: "A black armband is an insignia of mourning." "Flags flying above the building are insignia (or insignias) of national identity and pride." In Latin, the singular form is insigne, a form occasionally appearing in English: "The official wore the insigne of his position." General use suggests that insignia be employed in both singular and plural functions and that insigne, if used at all, should appear only as a singular.
Dictionary of problem words and expressions. Harry Shaw. 1975.