How To Handle Unique Constraint Violation

I've set up Oracle Streams replication between multiple N-way zones like this:

CODE------------------------------
Nway1 |Intersect| Nway2
DB1 --|-- DB3 --|-- DB5              DB1, DB2, DB3, DB4 - N-way1
  |   |    |    |    |               DB3, DB4, DB5, DB6 - N-way2
DB2 --|-- DB4 --|-- DB6              DB3, DB4 - intersection zone
      |         |  
-------------------------------

Physically DB1 .... DBN connected sequentially, so I want to prevent segmentation if some DB is unaccessible, but at the same time fight unneeded redundancy which uses too much link bandwidth to send N-1 LCR-s to all members of a single N-way group (so I want to split one big N-way zone into smaller ones and sequentially connect them into chain - it significantly reduces load on link if N is big enough (>10)). Also I want to have 2 DB in intersection zone to prevent single point of failure.

This scheme has one drawback - if change originated on DB3 or DB4, then it will be propagated (more correctly - applied and captured again) to DB5 and DB6 by both DB1 and DB2 (and, as far as I know, I have no means in capture rules to detect state of DB2 from DB1 and vise versa), so on DB5 and DB6 I get:

CODEORA-00001: unique constraint (DUMMYUSR.UNIQUE_RECORDS) violated error

I've set up standard conflict handler for apply process:

CODE

declare                                                                                        
    cols DBMS_UTILITY.NAME_ARRAY;                                                              
begin                                                                                          
    cols(1) := 'no';                                                                          
    cols(2) := 'name';                                                                        
    cols(3) := 'ddate';                                                                        
    dbms_apply_adm.set_update_conflict_handler(                                                
        object_name       => 'DUMMYUSR.DUMMYTBL',                                              
        method_name       => 'DISCARD',                                                        
        resolution_column => 'no',                                                            
        column_list       => cols                                                              
    );                                                                                        
end;

but it seems that it does not handle uniqueness conflicts. What is the best way to handle uniqueness conflict (is there a better way than to write custom error handler) and how serious is the impact on insert performance of having unique constraint and corresponding error handler. (In real world I will have to deal with tables with metainformation and without any keys).

Also, how to proceed with no error or raise exception from apply error handler with error that caused this handler to run? In oracle docs I can find only example that modifies LCR and runs lcr.EXECUTE(TRUE), but what to do if I don't want to reexecute LCR, but merely check error code and propagate error if it is not ORA-00001?