This paper presents a model of duopoly that incorporates economies of scale, product differentiation, and absolute advantages in cost or demand, aiming to develop a unified theory of entry barriers. The model considers both the Stackelberg and Nash equilibria, highlighting how different factors influence entry possibilities. The analysis shows that entry barriers can be effectively or ineffectively impeded depending on the underlying parameters of costs and demands. The model demonstrates that established firms can prevent entry by setting limit prices or by exercising monopoly power, while also showing that entry may be more difficult when fixed costs are high or when product differentiation is low. The paper also discusses the implications of these findings for industrial economics, emphasizing the importance of distinguishing between absolute advantages and product differentiation in understanding entry barriers. The model is applied to a specific case with linear demand and cost functions, illustrating how changes in parameters affect entry possibilities. The paper concludes that the effects of product differentiation on entry barriers are counterintuitive and that further research is needed to fully understand the role of product differentiation in industrial economics.This paper presents a model of duopoly that incorporates economies of scale, product differentiation, and absolute advantages in cost or demand, aiming to develop a unified theory of entry barriers. The model considers both the Stackelberg and Nash equilibria, highlighting how different factors influence entry possibilities. The analysis shows that entry barriers can be effectively or ineffectively impeded depending on the underlying parameters of costs and demands. The model demonstrates that established firms can prevent entry by setting limit prices or by exercising monopoly power, while also showing that entry may be more difficult when fixed costs are high or when product differentiation is low. The paper also discusses the implications of these findings for industrial economics, emphasizing the importance of distinguishing between absolute advantages and product differentiation in understanding entry barriers. The model is applied to a specific case with linear demand and cost functions, illustrating how changes in parameters affect entry possibilities. The paper concludes that the effects of product differentiation on entry barriers are counterintuitive and that further research is needed to fully understand the role of product differentiation in industrial economics.