If you put a siren from the siren mod and a stereo on the same vehicle it will cause a conflict that prevents the stereo from being properly controlled and can duplicate the audio.  

Your plugin appears to use code from the siren mod and that plugin has some kind of serious flaw that crashes servers on bootup if you enable the feature on boats.  The text it spits out is huge but here is a sample of it.

Failed to call hook 'OnEntitySpawned' on plugin 'CarRadio v1.0.2' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.CarRadio.Message (Oxide.Core.Libraries.Covalence.IPlayer aPlayer, System.String anI18nKey, System.Object[] someArgs) [0x00000] in <9f12a0d964be44cbb588e41f06060e3b>:0
  at Oxide.Plugins.CarRadio.AttachRadios (BaseVehicle aVehicle, Oxide.Plugins.CarRadio+Radio aRadio, Oxide.Plugins.CarRadio+RadioController+States anInitialState, Oxide.Core.Libraries.Covalence.IPlayer aPlayer) [0x000c3] in <9f12a0d964be44cbb588e41f06060e3b>:0
  at Oxide.Plugins.CarRadio.OnEntitySpawned (BaseVehicle aVehicle) [0x0006b] in <9f12a0d964be44cbb588e41f06060e3b>:0
  at Oxide.Plugins.CarRadio.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x004cc] in <9f12a0d964be44cbb588e41f06060e3b>:0
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <60c318df79ed41688ea59335e48d61ad>:0
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <12678b905a6d43c3a9cc366104306651>:0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <12678b905a6d43c3a9cc366104306651>:0
Failed to call hook 'OnEntitySpawned' on plugin 'CarRadio v1.0.2' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.CarRadio.Message (Oxide.Core.Libraries.Covalence.IPlayer aPlayer, System.String anI18nKey, System.Object[] someArgs) [0x00000] in <9f12a0d964be44cbb588e41f06060e3b>:0
  at Oxide.Plugins.CarRadio.AttachRadios (BaseVehicle aVehicle, Oxide.Plugins.CarRadio+Radio aRadio, Oxide.Plugins.CarRadio+RadioController+States anInitialState, Oxide.Core.Libraries.Covalence.IPlayer aPlayer) [0x000c3] in <9f12a0d964be44cbb588e41f06060e3b>:0

It seems the duplicate audio is a flaw with the plugin itself and not with the siren mod. Hitting the button only turns on the default first radio station.  And if you turn on a custom radio station on the boombox itself it duplicates and makes two stations play at once.