I've been building some native modules in order to improve my React Native app's performance.
I've learned how to build a native module with JSI bindings, which let you pass binary data as ArrayBuffer from JS to C++ so you can handle strings with NULL characters safely and process large data efficiently.
Here are two modules I made:

Now, it's time to integrate them with my app.
But I encountered an error when compiling the Android app:

More than one file was found with OS independent path 'lib/arm64-v8a/libcpp.so'

I confirmed each module works correctly on a separate project, so I suspected my project setting is wrong.
Checked build.gradle and app/build.gradle, but they look fine. Interesting.
Then, I found that it doesn't occur when I only installed one of them to my project.
So, using two causes the error. I looked closely at the error message. It says "More than one file" and both depend on "libcpp.so".
Well, where did "libcpp.so" come from? It looks like a std lib but no. I grepped the modules but couldn't find such build instruction.
I thought that it's the name of the shared libraries that my modules derive, and my project try to link them, but the name is duplicate.

react-native-quick-md5/android/CMakeLists.txt looks like so:

cmake_minimum_required(VERSION 3.4.1)

set (CMAKE_VERBOSE_MAKEFILE ON)
set (CMAKE_CXX_STANDARD 11)

add_library(cpp
  SHARED
  ../../react-native/ReactCommon/jsi/jsi/jsi.cpp
  ../cpp/quick-md5.cpp
  ../cpp/md5.cpp
  cpp-adapter.cpp
)

# Specifies a path to native header files.
include_directories(
  ../cpp
  ../../react-native/React
  ../../react-native/React/Base
  ../../react-native/ReactCommon/jsi
)

add_library adds a library to the project with specified name:

add_library(<name> [STATIC | SHARED | MODULE]
            [EXCLUDE_FROM_ALL]
            [<source>...])

Bingo. So, both modules are trying to add themselves as the same name: "cpp".
In android/src/main/java/com/reactnativequickmd5/QuickMd5Module.java, it calls System.loadLibrary("cpp"); like so:

class QuickMd5Module extends ReactContextBaseJavaModule {
  static {
    System.loadLibrary("cpp");
  }
}

Well, let's rename it.

diff --git a/android/CMakeLists.txt b/android/CMakeLists.txt
index 3ea329f..5540dbe 100644
--- a/android/CMakeLists.txt
+++ b/android/CMakeLists.txt
@@ -4,19 +4,20 @@ set (CMAKE_VERBOSE_MAKEFILE ON)
 set (CMAKE_CXX_STANDARD 11)

 include_directories(
-        ../cpp
-        ../../react-native/React
-        ../../react-native/React/Base
-        ../../react-native/ReactCommon/jsi
-)
+  ../cpp
+  ../../react-native/React
+  ../../react-native/React/Base
+  ../../react-native/ReactCommon/jsi
+  )

-add_library(cpp
-            SHARED
-            ../../react-native/ReactCommon/jsi/jsi/jsi.cpp
-            ../cpp/md5.cpp
-            ../cpp/md5.h
-            ../cpp/react-native-quick-md5.cpp
-            ../cpp/react-native-quick-md5.h
-            cpp-adapter.cpp
-)
+add_library(quickmd5 # Library name
+  SHARED
+  ../../react-native/ReactCommon/jsi/jsi/jsi.cpp
+  ../cpp/md5.cpp
+  ../cpp/md5.h
+  ../cpp/react-native-quick-md5.cpp
+  ../cpp/react-native-quick-md5.h
+  cpp-adapter.cpp
+  )

+target_link_libraries(quickmd5)
diff --git a/android/src/main/java/com/reactnativequickmd5/QuickMd5Module.java b/android/src/main/java/com/reactnativequickmd5/QuickMd5Module.java
index 8c9ce9a..223d319 100644
--- a/android/src/main/java/com/reactnativequickmd5/QuickMd5Module.java
+++ b/android/src/main/java/com/reactnativequickmd5/QuickMd5Module.java
@@ -8,7 +8,7 @@ import com.facebook.react.bridge.ReactContextBaseJavaModule;

 class QuickMd5Module extends ReactContextBaseJavaModule {
   static {
-    System.loadLibrary("cpp");
+    System.loadLibrary("quickmd5");
   }

It worked!
Hope it helps :)

How to fix: More than one file was found with OS independent path 'lib/arm64-v8a/libcpp.so'