Hi @AjayV,
Not sure how PowerBI works when creating relations and why it did not show duplicates, in QuickSight, they are joins between the tables, i you have a relation then your resulting dataset will have the data depending on the kind of join, inner, left, right, outter.
Here you can find more information on joining data in QuickSight
To perfrom RLS in different datasets, you need the column you will use for RLS in each of the datasets and have that applied in each dataset.
It is hard to advise any further without looking at the data model. It is possible that the data model you have might not be useful for the visualizations you want and you might need to do some more previous data processing than just joining tables.