Duplicate post problems

I have just imported 3,924 individual posts (no duplicates) into a new and empty WP install. But I have ended up with 4,130 posts, many duplicates but also many missing posts.

I did the import in 4 batches, of about 1,000 each.

Status reports included:
1:
I did not check for duplicate posts, and did not record the report.

2:
16 records are successfully Imported.993 duplicate records found.
Post count increased by 1,002, from a batch containing 999 posts.

3:
83 records are successfully Imported.917 duplicate records found.
Post count increased by 1,296 posts, from a batch containing 999 posts.

4:
I forgot to check for duplicate posts.
The final import of 986 posts increased my post count by only 806.

I do seem to have many duplicated posts, but none were duplicated in the csv files.
I also seem to have at least 180 missing posts, from the last batch alone.

I am not sure what is happening. Any ideas ?

http://ww.wp.xz.cn/plugins/wp-ultimate-csv-importer/