An outer join doesnt determine unique records yes or no... it just states you want everything from the 'left; matching & nonmatching values . Just behavior as expected. However it is possible to do this in tMap to get a first / unique match.
Maybe this helps: https://help.talend.com/reader/4sGi6uta35W98meBu6klVg/~1Q8eE0O0Ru1ZGqty7lcJg
@muralam,since you can restrict at source side to eliminate duplicates while featching from Lookups and other other one you can change the Match Model from All matches to First matche to get First record or Unique match to get the Last mached record.
If you use Fist or Unique match you wil get only 3927 .records.