One way in which monopolistic competition and monopoly market structures are the same is that both involve firms that have some degree of market power.
What happens in monopolistic competition ?
In a monopolistic competition market, firms have some control over the price of their differentiated products because they have brand loyalty and can differentiate their products from competitors, although the degree of control may be limited.
In a monopoly market, a single firm has substantial control over the price because it is the sole provider of a particular product or service.
One way in which they are different is in the number of firms operating in the market. In monopolistic competition, there are many firms competing with each other, each offering slightly differentiated products.