Search while indexing

[sonfon]

Dear All,
    Now, I'm considering to build index for my application with lucene. 
However, as the document sources I'm going to index has many duplications, so 
before adding a document to an IndexWriter, I hope search in the index database 
first to see if a same document copy has already been added. I used 
IndexSearcher to search the same Dir while IndexWriter writing to it. However, 
it seem that IndexSearcher returned no result though I'm sure there are 
duplicate copies indexed already. And after the indexing procedure, I can get 
the search results, so I'm sure I didn't write the wrong code. Anyone could 
offer some help? Some example codes are appreciated.
    Best
Wishes.