(1) Background: Brazil has a universal public healthcare system, but individuals can still opt to buy private health insurance and/or pay out-of-pocket for healthcare. Past research suggests that Brazilians make combined use of public and private services, possibly causing double costs. This study aims to describe this dual use and assess its relationship with socioeconomic status (SES). (2) Methods: We calculated survey-weighted population estimates and descriptive statistics, and built a survey-weighted logistic regression model to explore the effect of SES on dual use of healthcare, including demographic characteristics and other variables related to healthcare need and use as additional explanatory variables using data from the 2019 Brazilian National Health Survey. (3) Results: An estimated 39,039,016 ( = 46,914; 18.6%) persons sought care in the two weeks before the survey, of which 5,576,216 were dual users ( = 6484; 14.7%). Dual use happened both in the direction of public to private ( = 4628; 67.3%), and of private to public ( = 1855; 32.7%). Higher income had a significant effect on dual use ( < 0.0001), suggesting a dose-response relationship, even after controlling for confounders. Significant effects were also found for region ( < 0.0001) and usual source of care (USC) ( < 0.0001). (4) Conclusion: A large number of Brazilians are seeking care from a source different than their regular system. Higher SES, region, and USC are associated factors, possibly leading to more health inequity. Due to its high prevalence and important implications, more research is warranted to illuminate the main causes of dual use.