How to solve "duplicate symbol" when added Firebase in Xcode

I had the same issue. It seems that the -all_load flag in the Other Linker Flags is causing this.

There was a Firebase build issue introduced in 3.11.1 that will be fixed in 3.13.0 that can cause the duplicate symbol problem. In the meantime, a workaround is to back up to Firebase 3.11.0, by annotating any Firebase subspecs in your podfile with an explicit version number, like :

pod 'Firebase/Analytics', '3.11.0'