Americans pay more in tax towards public healthcare than we do in some European countries. Then they pay again.

E.g. the UK NHS costs less per capita than Medicare + Medicaid costs per capita (not per user), despite the former providing universal service. Partly explained by artificial restrictions on Medicare limiting their ability to negotiate price - the "free" market is intentionally prevented from functioning. It's pretty much corporate welfare paid for by regular tax payers. I never understand why Americans tolerate this.

The proportion who pays twice in the UK is around 10% who opt for private insurance on top. The proportion who pays twice in the US is every tax payer.

Total healthcare spend per capita (public + private) tells a pretty clear picture, where costs of healthcare in the US is totally out of control.