Hello Asim,
if you have R1 and R6 to generate a default route of type E1 the routers in the OSPF domain will receive both LSA type 5 and will use the one with a lower overall metric:
O E1 : seed metric + metric to reach ASBR
to get it just add metric-type 1 on the default-originate command inside router ospf process on R1 and R6.
Depending on your topologies you can have different results: half routers going through R1, some routers installing both routes if they have equal cost from their own point of view.
You cannot however filter LSA type 5 in OSPF: you can use distribute-list to avoid to have a prefix installed in the local routing table but this will not stop LSA flooding in OSPF domain.
You can also play with different seed metric to get a better result if needed.
Hope to help
Giuseppe