Although they are very different species, fish and dolphins (mammals) look similar. Their body shape is highly streamlined: both have fins and a tail. Evolution appears to have repeated itself in producing these two animals that are formed alike.
Similar adaptations occur quite frequently, however, but none of them are due to "repeating evolution". When distantly related lineages such as fish, extinct reptiles such as ichthyosaurs, and mammals evolve so that they are superficially similar in external appearance, they are said to be converging, and the trend is called convergent evolution.
Subjected to the same environmental forces, fish, aquatic reptiles and aquatic mammals independently evolved similar shapes because a streamlined body is the most efficient way of moving through the dense medium of water.
Traits that produced smoother outlines arose on their own, were favored by natural selection in each group, and were passed on, with further and further modification in each line of descent.
Gradually sharks, ichthyosaurs, and dolphins came to look alike because natural selection favored one particular shape over all others for rapid movement through the seas.