It's definitely not a duopoly and if it were it definitely wouldn't include Zenni. Here's one ranking of the top eyeglass retailers in 2015[0]. Another source says Zenni Optical had gross revenue of $214MM in 2018[1], so maybe they should be on that list but they'd be behind Warby Parker let alone some of the others.

[0] https://www.statista.com/statistics/256664/sales-of-the-lead... [1] https://www.marketwatch.com/press-release/zenni-optical-cele...