Environment

• react@16.12.0
• react-dom@16.12.0
• react-scripts@3.0.1
• styled-components@5.0.1

⬇️ see codesandbox example.

Reproduction

• codesandbox - buggy w/ styled-components@5.0.1
• codesandbox - working w/ styled-components@4.4.1

Steps to reproduce

Open the provided codesandbox and inspect the styling of either of the headlines. The h1 (or h2) styling is applied twice because it was added twice to the document.

If you downgrade styled-components (see codesandbox) to the latest 4.x.x version and inspect the elements again, everything is fine.

Expected Behavior

Global style is added only once.

Actual Behavior

Global style is added twice to the document (also in a SSR scenario, which happens in one of our apps right now).

Screenshots

Bug with version 5.0.1

Working w/ version 4.4.1