beis hamikdash - Was there a duplicate of the Aron Hakodesh?

The Rambam writes in Hilchos Klei Mikdash (7:19) that all the vessels in the Beis Hamikdash had duplicates (and duplicates of the duplicates) just in case a vessel became impure. Did the Aron Hakodesh have a duplicate? I always assumed that Aron was unique and there was only one of them. In addition, since it was in the Kodesh Hakedoshim it was probably very unlikely to be touched by an impure person.