For nearly as long as mankind has worn fancy things, someone out there has been trying to copy those things.

Ancient artisans cribbed from each other’s designs. Victorian hustlers duped diamond jewelry with shimmery paste. Christian Dior, the godfather of high fashion himself, was believed to sell his own designs to lesser known producers, who could make cheaper versions.

For the modern athleisure giant Lululemon, however, the duping has gone too far.

In a lawsuit filed in California on June 25, the company accused the wholesale retailer Costco of selling knockoff sweatshirts, jackets and other activewear at a fraction of what Lululemon charges.

Successfully suing over knockoffs is notoriously difficult for fashion brands and designers, in part because copyright laws are often vague, experts say. Lululemon’s case seeks restitution and payment from Costco, which it said profited off Lululemon’s “sweat equity” and damaged the Lululemon brand. Costco did not immediately respond to a request for comment.

Lululemon said Costco is selling knockoffs of its Scuba hoodies and sweatshirts, Define jackets and popular ABC pants for men.

Using design images and side-by-side comparisons, Lululemon, in a 49-page court filing, argued that Costco sold jackets and hoodies — made by clothing manufacturers such as Danskin, Spyder and Jockey — using terms in its marketing similar to what Lululemon uses.

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