If you are running a batch task for some net build and the build uses dynamic
code compilation (like ASP.Net, xsl transformation (MbUnit reporting)) then you
can get the following error stack (snippet):

Item has already been added. Key in dictionary: 'path' Key being added: 'path'
[System.ArgumentException: Item has already been added. Key in dictionary:
'path' Key being added: 'path'
at System.Collections.Hashtable.Insert(Object key, Object nvalue, Boolean add)
at System.Collections.Hashtable.Add(Object key, Object value)
at System.Collections.Specialized.StringDictionary.Add(String key, String value)
at
System.CodeDom.Compiler.Executor.ExecWaitWithCaptureUnimpersonated(SafeUserTokenHandle
userToken, String cmd, String currentDir, TempFileCollection tempFiles, String&
outputName, String& errorName, String trueCmdLine)
at System.CodeDom.Compiler.Executor.ExecWaitWithCapture(SafeUserTokenHandle
userToken, String cmd, String currentDir, TempFileCollection tempFiles, String&
outputName, String& errorName, String trueCmdLine)
at Microsoft.CSharp.CSharpCodeGenerator.Compile(CompilerParameters options,
String compilerDirectory, String compilerExe, String arguments, String&
outputFile, Int32& nativeReturnValue, String trueArgs)
at Microsoft.CSharp.CSharpCodeGenerator.FromFileBatch(CompilerParameters
options, String[] fileNames)
at Microsoft.CSharp.CSharpCodeGenerator.FromSourceBatch(CompilerParameters
options, String[] sources)
at
Microsoft.CSharp.CSharpCodeGenerator.System.CodeDom.Compiler.ICodeCompiler.CompileAssemblyFromSourceBatch(CompilerParameters
options, String[] sources)
at
System.CodeDom.Compiler.CodeDomProvider.CompileAssemblyFromSource(CompilerParameters
options, String[] sources)
...

This is caused (in my case) by a double entry for the path environment variable.
I printed out the Net System.Environment while running under hudson (snippet):

...
Path:C:\WINNT\system32;C:\WINNT;C:\WINNT\System32\Wbem;C:\DrDOC\SYSTEM;C:\Programme\Microsoft
Visual Studio\Common\VSS\win32;C:\Programme\Microsoft SQL
Server\80\Tools\BINN;C:\Programme\Support
Tools\;C:\Programme\CVSNT\;C:\Programme\apache-ant\bin\;c:\Programme\svn\bin
SVN_REVISION:104
USERDNSDOMAIN:xxx.de
BUILD_ID:2007-10-31_10-05-38
PATH:C:\Programme\Java\jdk1.5.0_05\bin
...

For the base problem please see the following forum rss:

http://forums.microsoft.com/msdn/rss.aspx?postid=231948&forumid=18&siteid=1