FML Security Error

If you have Forge installed in multiple locations, this can result in an FML security error, similiar to the one printed below. For example, this can occur if you have a forge-14.23.4.2707.jar jar in your primary directory, and then the same jar at /mods/forge-14.23.4.2707.jar.

[main/ERROR] [LaunchWrapper]: Unable to launch java.lang.RuntimeException: FML was unable to install the security manager. The game will not start at net.minecraftforge.fml.common.launcher.FMLTweaker.<init>(FMLTweaker.java:58) ~[forge-14.23.4.2707.jar:?] at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:1.8.0_171] at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[?:1.8.0_171] at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[?:1.8.0_171] at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[?:1.8.0_171] at java.lang.Class.newInstance(Class.java:442) ~[?:1.8.0_171] at net.minecraft.launchwrapper.Launch.launch(Launch.java:98) [launchwrapper-1.12.jar:?] at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_171] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_171] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_171] at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_171] at net.minecraftforge.fml.relauncher.ServerLaunchWrapper.run(ServerLaunchWrapper.java:62) [forge-14.23.4.2707.jar:?] at net.minecraftforge.fml.relauncher.ServerLaunchWrapper.main(ServerLaunchWrapper.java:31) [forge-14.23.4.2707.jar:?] Caused by: java.lang.SecurityException: Cannot replace the FML security manager at net.minecraftforge.fml.relauncher.FMLSecurityManager.checkPermission(FMLSecurityManager.java:54) ~[forge-14.23.4.2707.jar:?] at java.lang.System.setSecurityManager0(System.java:300) ~[?:1.8.0_171] at java.lang.System.setSecurityManager(System.java:291) ~[?:1.8.0_171] at net.minecraftforge.fml.common.launcher.FMLTweaker.<init>(FMLTweaker.java:54) ~[forge-14.23.4.2707.jar:?] ... 13 more

To resolve this error, remove the duplicate Forge jar, generally found in your /mods directory.