Когда я brew update нашел https://github.com/Homebrew/homebrew-dupes/ не найден:
brew update
https://github.com/Homebrew/homebrew-dupes/
delo-MacBook-Pro:~ ldl$ brew --version Homebrew 2.2.2 Homebrew/homebrew-core (git revision 9e4997; last commit 2019-07-30) Homebrew/homebrew-cask (git revision 4b0e1; last commit 2019-12-27) delo-MacBook-Pro:~ ldl$ brew update remote: Repository not found. fatal: repository 'https://github.com/Homebrew/homebrew-dupes/' not found
Попробуйте это