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.
