Duplicate data returned - Mailing list pgsql-jdbc

I experience a situation for the same select statement having duplicate data returned using jdbc of pgsql, while only
one 
single set date returned  on the pqsl window. I need to add the "distinct" in the select statement to have a single set

data returned.

Is the advicable practise to have the 'distinct' key word on every selection statement?

Thanks,

Vernon