Given a graph G = (V, E), the dominating set problem asks for a minimum subset of vertices D ⊆ V such that every vertex u ∈ V \D is adjacent to at least one vertex v ∈ D. That is, the set D satisfies the condition that |NIn this paper, we study two variants of the classical dominating set problem: k-tuple dominating set (k-DS) problem and Liar's dominating set (LDS) problem, and obtain several algorithmic and hardness results.On the algorithmic side, we present a constant factor ( 11 2 )-approximation algorithm for the Liar's dominating set problem on unit disk graphs. Then, we obtain a PTAS for the k-tuple dominating set problem on unit disk graphs. On the hardness side, we show a Ω(n 2 ) bits lower bound for the space complexity of any (randomized) streaming algorithm for Liar's dominating set problem as well as for the k-tuple dominating set problem. Furthermore, we prove that the Liar's dominating set problem on bipartite graphs is W[2]-hard.