about "Oracle recommends that unique indexes be created explicitly"

The book of "oracle concept" says
   
  Oracle recommends that unique indexes be created explicitly, using CREATE 
UNIQUE INDEX. Creating unique indexes through a primary key or unique 
constraint is not guaranteed to create a new index, and the index they create 
is not guaranteed to be a unique index.
   
  So under what condition will the created index through primary key or unique 
constraint not be a unique index?
   

                
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