Learn Russian: Мы случайно купили один и тот же костюм! - We accidentally bought the exact same suit!

Both phrases mean 'the exact same', but один и тот же is particularly natural when multiple people independently choose or end up with an identical type of thing (like two people buying the same suit). Тот же самый is often preferred when comparing a specific object to itself over time (e.g., 'He wore the exact same suit as yesterday'). However, in many contexts, they are interchangeable.

How is the phrase один и тот же declined in this sentence?

It is in the accusative case because it modifies the direct object, костюм. Because костюм is a masculine, inanimate noun, its accusative form looks exactly like the nominative. When you decline this phrase in other cases, remember that both один and тот must change to match the noun's gender, case, and number, while и and же stay exactly as they are.

Does the word костюм only mean a business suit?

By itself, костюм usually means a business suit (a jacket and trousers or a skirt) or a theatrical costume. However, it is a versatile word used in everyday combinations for other matching outfits, such as спортивный костюм (tracksuit).

Why is the perfective verb купили used here instead of the imperfective покупали?