How to update a Lucene.NET index?

I'm developing a Desktop Search Engine in Visual Basic 9 (VS2008) using Lucene.NET (v2.0).

I use the following code to initialize the IndexWriter

Private writer As IndexWriter

writer = New IndexWriter(indexDirectory, New StandardAnalyzer(), False)

writer.SetUseCompoundFile(True)

If I select the same document folder (containing files to be indexed) twice, two different entries for each file in that document folder are created in the index.

I want the IndexWriter to discard any files that are already present in the Index.

What should I do to ensure this?