First of all, this react native error can be caused by not only with RNFirebase library but also with other native libraries. This react native module override issue can be solved as given below.
Navigate to android/app/src/main/java/com/yourproject/MainApplication.java and check if you have used RNFirebasePackage() twice inside it. Maintain only one RNFirebasePackage() inside MainApplication.java file to fix the issue.
This issue can be happened by any native library, all you need is to verify the package is added only once in MainApplication.java file.