How can I quit this: "Unique " to my table?

Hello pgsql-sql,
 When my table was create I wrote this:
 MyDatabase=# create table b_prestamo(no_inventa char(16) unique not null, cve_area char(10), date1 date); I need now
thatmy table can accept repeated records, but not Null. How can I do that? SomeBody can help me please? My table has
informationat the moment,what can I do for maintain it?  
 

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Best regards,Luis                          mailto:ksql2002@yahoo.com.mx