Cheaper look-alikes have been around for years, but social media has made them more popular. Dupes can move into shaky legal territory if concerns arise around consumer confusion or patented designs.

Lululemon alleges that Costco has stolen the intellectual property of its Scuba hoodies and sweatshirts, Define jackets and ABC pants.

Lululemon refers to itself in the lawsuit as a "path-breaking" apparel company that has set a new standard for performance athletic wear, adding that its "construction techniques" and "strategic fits" help it stand out to consumers. Reached for comment, the Vancouver, Canada-based company said it fiercely defends its products.