**Main Insights:**
- The startup Double aims to offer index investing with a competitive 0% expense ratio, challenging established players like Vanguard and Fidelity.
- User comments raise concerns about spread costs associated with buying individual stocks versus established ETFs, questioning whether the proposed low-cost model truly benefits investors.
- Skepticism exists regarding the company's long-term viability given their low fee structure and how they plan to generate revenue without compromising services.