steal someone's thunder

To upstage someone; to destroy the effect of what someone does or says by doing or saying the same thing first: “The Republicans stole the Democrats' thunder by including the most popular provisions of the Democratic proposal in their own bill.”

Use or appropriate another's idea, especially to one's advantage, as in It was Harold's idea but they stole his thunder and turned it into a massive advertising campaign without giving him credit. This idiom comes from an actual incident in which playwright and critic John Dennis (1657–1734) devised a “thunder machine” (by rattling a sheet of tin backstage) for his play, Appius and Virginia (1709), and a few days later discovered the same device being used in a performance of Macbeth, whereupon he declared, “They steal my thunder.”