steal one's thunder
steal one's thunder {v. phr.}
To do or say something, intentionally or not, that another person has planned to say or do.
Fred intended to nominate Bill for president, but John got up first and stole Fred's thunder.
Mary was going to sing "Oh! Susanna," but Ellen did it first and Mary said Ellen had stolen her thunder.
Smith heard that Jones was going to offer a new law which people wanted, so he himself proposed the law first, stealing Jones' thunder.
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Source: A Dictionary of American Idioms