Short answer: Yes. Fish oil is rich in Omega-3 fatty acids which are proven to be healthier than other forms of fat; however, someone who regularly eats fish would likely not experience any benefit to taking a fish oil supplement.
Longer answer: Scientists have known about the benefits of fish consumption for decades now after seeing the link between populations with a tradition of heavy fish eating and low rates of heart disease. Diets from the Mediterranean and Japan are weighted heavily toward fish as opposed to common diets from the United States, which tend to have lots of beef and carbohydrates. The FDA now recommends that people eat around 2-3 servings of fish a week, primarily for its association with lower risk of cardiovascular disease. The FDA explicitly mentions DHA and EPA (the active ingredients in fish oil) as the primary cause for benefits from fish. As with most things, too much of a good thing can be a bad thing, and an excess of fish oil has been shown to negatively affect the immune system and increase bleeding. In conclusion, if you already eat fish regularly, you would likely see no benefits and possibly some side effects. However, for people who are allergic to fish or don’t like seafood, fish oil supplements high in DHA and EPA can improve their cardiovascular health and eliminate Omega-3 deficiencies.