Consider that the relation has only one attribute $A$.
@shashin, Yes I agree that we can translate RA to sql.
But as far as I know RA query only takes relation as input and gives relation as output and this relation is set of tuples.
So I thought that relation should not include any duplicate.
@Vimal Patel - does this help ?
Formal query languages are based on mathematical relations. Thus no duplicates appear in relations.
As duplicate removal is expensive, SQL allows duplicates.
Here we are just told 'relations' - so we assumed a practical implementation that mostly allows duplicates. From a purely mathematical perspective - no duplicates. What say ?