In the tree reconciliation approach to infer the duplication history of a gene family, the gene (family) tree is compared to the corresponding species tree. Incomplete lineage sorting (ILS) gives rise to stochastic variation in the topology of a gene tree and hence likely introduces false duplication events when a tree reconciliation method is used. We quantify the effect of ILS on gene duplication inference in a species tree in terms of the expected number of false duplication events inferred from reconciling a random gene tree, which occurs with a probability predicted in coalescent theory, and the species tree. We computationally examine the relationship between the effect of ILS on duplication inference in a species tree and its topological parameters. Our findings suggest that ILS may cause non-negligible bias on duplication inference, particularly on an asymmetric species tree. Hence, when gene duplication is inferred via tree reconciliation or any other approach that takes gene tree topology into account, the ILS-induced bias should be examined cautiously.