We study how common ownership affects the magnitude and dynamics of investments in a duopoly. Followers exhibit less aggressive timing and quantity reactions because they internalize their effects on leaders. Leaders are therefore more likely to opt for a deterrence strategy, but their own internalization of followers softens their decisions. If firm roles are exogenous, high common ownership links lead to a relatively efficient staged investment outcome. Conversely, if firm roles are endogenous, high common ownership drives the winner of the preemption race to concede a “follower monopoly.” Our numerical analysis finds that common ownership is generally detrimental to consumer surplus and welfare.