The dupondius was a brass coin struck from a bronze alloy called orichalcum valued at 2 copper asses. The coin features the bust of Roma on the obverse and a six-spoked wheel on the reverse and was in circulation up until 300 AD.
Because Dupondii minted prior to and during the reign of Nero, and occasionally under later rulers, lack the radiate crown, it can be hard for collectors to distinguish between the as and the dupondius due to heavy patination which obscures the coin's original color.
However starting with Nero, the emperor is always shown wearing a radiate crown which makes it distinguishable from sestertii and asses.