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[v2] 2.0.0-17 SSR can contains conflicting keys

See original GitHub issue

Hi, I’m using styled-component with Next.js and found out the SSR in 2.0.0-17 can produce style elements with the same keys.

const main = sheet.collectStyles(<Main />);
const styleTags = sheet.getStyleElement();
console.log(styleTags);

next_1   | [ { '$$typeof': Symbol(react.element),
next_1   |     type: 'style',
next_1   |     key: 'fNItctcyArQf',
next_1   |     ref: null,
next_1   |     props:
next_1   |      { type: 'text/css',
next_1   |        'data-styled-components': 'fNItct cyArQf',
next_1   |        'data-styled-components-is-local': 'true',
next_1   |        dangerouslySetInnerHTML: [Object] },
next_1   |     _owner:
next_1   |      ReactCompositeComponentWrapper {...},
next_1   |     _store: {} },
next_1   |   { '$$typeof': Symbol(react.element),
next_1   |     type: 'style',
next_1   |     key: '',
next_1   |     ref: null,
next_1   |     props:
next_1   |      { type: 'text/css',
next_1   |        'data-styled-components': '',
next_1   |        'data-styled-components-is-local': 'false',
next_1   |        dangerouslySetInnerHTML: [Object] },
next_1   |     _owner:
next_1   |      ReactCompositeComponentWrapper {...},
next_1   |     _store: {} },
next_1   |   { '$$typeof': Symbol(react.element),
next_1   |     type: 'style',
next_1   |     key: 'boZsUk',
next_1   |     ref: null,
next_1   |     props:
next_1   |      { type: 'text/css',
next_1   |        'data-styled-components': 'boZsUk',
next_1   |        'data-styled-components-is-local': 'true',
next_1   |        dangerouslySetInnerHTML: [Object] },
next_1   |     _owner:
next_1   |      ReactCompositeComponentWrapper {...},
next_1   |     _store: {} },
next_1   |   { '$$typeof': Symbol(react.element),
next_1   |     type: 'style',
next_1   |     key: '',
next_1   |     ref: null,
next_1   |     props:
next_1   |      { type: 'text/css',
next_1   |        'data-styled-components': '',
next_1   |        'data-styled-components-is-local': 'false',
next_1   |        dangerouslySetInnerHTML: [Object] },
next_1   |     _owner:
next_1   |      ReactCompositeComponentWrapper {...},
next_1   |     _store: {} },
next_1   |   { '$$typeof': Symbol(react.element),
next_1   |     type: 'style',
next_1   |     key: 'fbMseBbPlOHciZIztWijwppWcYHBKnVUVurkOMPXzexiBkeKdYRFdpNGlqiGXYPIfWzfGwdWMukr',
next_1   |     ref: null,
next_1   |     props:
next_1   |      { type: 'text/css',
next_1   |        'data-styled-components': 'fbMseB bPlOHc iZIztW ijwppW cYHBKn VUVur kOMPXz exiBke KdYRF dpNGlq iGXYPI fWzfGw dWMukr',
next_1   |        'data-styled-components-is-local': 'true',
next_1   |        dangerouslySetInnerHTML: [Object] },
next_1   |     _owner:
next_1   |      ReactCompositeComponentWrapper {...},
next_1   |     _store: {} } ]

next_1   | Warning: flattenChildren(...): Encountered two children with the same key, `5:1:$`. Child keys must be unique; when two children share a key, only the first child will be used.
next_1   |     in head (created by Head)
next_1   |     in Head (at _document.js?entry:27)
next_1   |     in html (at _document.js?entry:26)
next_1   |     in MyDocument

As you can see in the logs, 2 styles elements have empty string keys. I’m 100% sure why, but I think these are caused by multiple calls to injectGlobal.