Let R = F p m [u]/ u 3 be the finite commutative chain ring, where p is a prime, m is a positive integer and F p m is the finite field with p m elements. In this paper, we determine all repeatedroot constacyclic codes of arbitrary lengths over R and their dual codes. We also determine the number of codewords in each repeated-root constacyclic code over R. We also obtain Hamming distances, RT distances, RT weight distributions and ranks (i.e., cardinalities of minimal generating sets) of some repeatedroot constacyclic codes over R. Using these results, we also identify some isodual and maximum distance separable (MDS) constacyclic codes over R with respect to the Hamming and RT metrics. INDEX TERMS Cyclic codes; Local rings; Negacyclic codes; Optimal codes.