L.L. Bean struggled to keep up with demand for its popular duck boots last holiday season and wound up with 100,000 back-ordered pairs and no way to make them fast enough. Since then, the Maine shoemaker has shored up its manufacturing operations so it wouldn’t happen again.

It’s happening again.

Bean operates two factories, both in its home state, where fewer than 500 workers put together the duck boots and other items, like tote bags. It has hired 100 new boot makers and continues to add more. It turned on a second molding injection machine in June, a $1 million investment it made last winter, to make those squeaky rubber bottoms faster. It continues to run a third shift, which it put in place during the great boot rush of 2014.

It’s only September, and some styles and sizes of Bean’s duck boots are already on back order.

“We’re making them literally as fast as we can,” said Mac McKeever, a spokesman for L.L. Bean. “It’s not something we can just stamp out.”

The scarcity itself may be helping L.L. Bean, by setting it apart from competitors that make similar styles. Many fashion labels now sell duck boots, and the century-old, privately owned Bean counts on its legacy and craftsmanship for an air of exclusivity. That’s one reason it doesn’t outsource manufacturing to other factories, as much of the apparel and footwear industries do.

Boots popped up on eBay last year with sellers asking for twice the price or more. You can’t buy that kind of marketing. Then again, the shortage frustrated some customers who couldn’t get their boots until after the holidays. As shoppers rushed to score a pair of ducks before last year’s first snowfall, Bean couldn’t keep up, and didn’t clear the backlog till July.

Made of leather and rubber, L.L. Bean’s sturdy footwear was once better known for its popularity among hardy outdoors people than stylish city dwellers. Since 2009, the duck boot style has caught on in the fashion industry, one of many vintage brands that went from utilitarian staple to flashy fashion statement. “Call it heritage chic,” the New York Times declared at the time. Sales shot up for items like Canada Goose winter parkas and Sorel snow boots.

The company sold about 450,000 pairs of the boots last year and expects to sell 500,000 or more this year, McKeever said. If a particularly harsh winter hits the U.S., the backlog could get even worse, he said. For now, Bean is focused on cranking out the traditional duck boot style, with plans to devote more resources to shearling and waterproof styles as winter hits.

“We realize we could outsource, but that will never happen,” McKeever said. “The boots have been hand-sewn in Maine by our own skilled boot workers, and they always will be.”