Couldn't find this error anywhere on the forums, so I wanted to check if anyone has seen it before I submit a bug report.

It's only happening in IE8 for me, not Firefox, when running the web player and is happening with a new project using the sample scene provided by default. It appears to be something environment related but I've double checked and haven't installed anything in a few weeks and am not sure where or how to modify the MONO_PATH variable. As soon as the web player loads and starts the Unity splash screen it crashes. A reinstall of the web player didn't help, I'll move on to uninstalling Unity next. Web player log has:

100608 10:24:16 ------------------------------------------------------------
100608 10:24:16 Instance starting, version 2.6.1f3_31223
100608 10:24:16 Datafile file:///C:/Projects/Dev/Unity/Empty/Empty/Empty.unity3d
web: player version 2.6.1f3
Direct3D:
    Version:  Direct3D 9.0c [igdumdx32.dll 7.15.10.1527]
    Renderer: Mobile Intel(R) 4 Series Express Chipset Family
    Vendor:   Intel
    VRAM:     1545 MB
D3D: disabling fullscreen on Vista+IE8
web: load mono

Here is the app crash report in IE8

Could not start compilationArgumentException: Key duplication when adding: MONO_PATH
System.Collections.Hashtable.PutImpl (System.Object key, System.Object value, Boolean overwrite) 
System.Collections.Hashtable.Add (System.Object key, System.Object value) 
System.Collections.Specialized.ProcessStringDictionary.Add (System.String key, System.String value) 
UnityEditor.CompilerHelper+BackgroundProcess..ctor (System.String command, System.Collections.ArrayList arguments, Boolean supportResponseFile) 
UnityEngine.ScriptCompilers.UnityScriptCompiler.BeginCompiling (System.String[] sources, System.String[] references, System.String[] defines, System.String outputFile) [0x00000]