fix: throw when refresh runtime is loaded twice (fix #101)

[ArnaudBarre]

Fix #101

[sapphi-red]

I’m afraid this doesn't work.
preambleCode is injected by transformIndexHtml that doesn't run with backend integration.

vite-plugin-react/packages/plugin-react/src/index.ts

Lines 369 to 379 in 92162c6

	transformIndexHtml() {
	if (!skipFastRefresh)
	return [
	{
	tag: 'script',
	attrs: { type: 'module' },
	children: preambleCode.replace(`__BASE__`, devBase),
	},
	]
	},
	}

[ArnaudBarre]

Oh yeah true this should be in the runtime code so!