Can firms in a perfectly competitive market make supernormal profits?

Firms in a perfectly competitive market can make supernormal profits but only in the short run. Supernormal profit is made where average revenue exceeds average cost. In a perfectly competitive market, firms are price takers which means that they have no bearing on the market price. Thus, they are essentially facing infinite demand at the market price which means that average revenue is not only equal to marginal cost, but is also constant and equal to the market price. Costs on the other hand are kept as low as possible to avoid being priced out of the market. Output is determined by the profit maximising condition i.e. where marginal revenue is equal to marginal cost. This could potentially lead you to a situation where average cost exceeds average revenue but this will only be the case in the short run. Perfectly competitive markets have no barriers to entry and there is perfect knowledge in the market. If non-incumbent firms can see that there are supernormal profits being made in the market, they will be incentivised to join. The absence of barriers to entry means that they can do so with ease. However, when new firms join the market, industry supply increases which pushes the market price down and reduces the amount of supernormal profit being made as average revenue will decrease also. Firms will keep entering the market until there is no more incentive to do so, i.e. when only normal profit is being made. In the long run, the equilibrium will settle where only normal profit is made and thus competitive pressures ensure that supernormal profit for a firm in a perfectly competitive market is only a short run phenomenon.