one-trick pony
Helen: Did you know that Johann is a local champion at table tennis?
Karen: Yes, but he has a boring job at the bank, and table tennis is his only hobby. He's just a one-trick pony.
Explanation: When we say that someone is a "one-trick pony" it means they are like a small horse in the circus that can only do one thing to impress people. It means that they don't a range of talents. They have just one special skill, and it's not so impressive.
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