Like Simon mentioned you might want to create a document identifier or UUID - if you don't have one already and use this code snippet to check if doc exists:
string doc_id = "1234567";
Term idTerm = new Term(Fields.DOCID_FIELD,doc_id);
if (mSearcher.docFreq(idTerm) > 0) {
//mIndexWriter.updateDocument(idTerm,doc);
//This document exists hence skip
} else {
mIndexWriter.addDocument(doc);
}
