A growing number of digital platforms operate in a dual mode: running marketplaces for third-party products, while selling their own products on those marketplaces. We build a model to explore the implications of this controversial practice. We analyze the tradeoffs that arise from a regulatory ban on the dual mode, showing how such a ban can harm consumer surplus and welfare even when the platform would otherwise engage in product imitation and self-preferencing. In the empirically most relevant scenarios, policies that prevent platform imitation and self-preferencing generate better outcomes than an outright ban on the dual mode.