SQL Update Question

I have written a Mass Change Function for my application using SQL, the users 
specifies parameters and the program updates the master and transaction files, 
by building up SQL Update statements , It has facilities to change keys on a 
master file. I am looking for a way to 1) Stop it failing when the
update, causes a duplicate key.  2) Delete the records to be updated that 
result in a duplicate.

E.g.    Rcd 1   Key 1 = A   Key 2 = B   Key 3=C  Key 4=X
       Rcd 2   Key 2 = A   Key 2 = B   key 3=D  Key 4=X
       Rcd 3   Key 2 = A   Key 2 = B   key 3=C  Key 4=Y

User has requested to change Key3 from c to d. for All records with key 1=A, 
2=B and 3=C This will result in a duplicate rcd and the update will fail, I 
would like to delete rcd 1 if it results in a duplicate.


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