Generating Unique Random Values for Array.
 [Supported in IEEE 1800-2012] 


1). Randomizing all 32 members of array ,each member with 16 bit width

   class  random_unique_array_c;
       rand bit [15:0] uniq_arr [31:0];
       constraint  unique_cnstnt {unique {uniq_arr };}
   endcalss : random_unique_array_c


2). Randomizing all 32 members of array ,each member with 16 bit width, with exclusion of  some values.

   class  random_unique_array_c;
       rand bit [15:0] uniq_arr [31:0];
               bit [15:0] exclude_val [ ] = {3,6,9};
       constraint  unique_cnstnt {unique {uniq_arr,exclude_val,12 };}   //uniq_arr will not have values
   endcalss : random_unique_array_c                                                     //3,6,9,12



For more info see IEEE 1800-2012 LRM  section  18.5.5 "Uniqueness constraints"