Clarify handling of duplicated keys

Author: achow101

bip-0174.mediawiki

@@ -260,10 +260,9 @@ pass those key-value pairs through when re-serializing the transaction.
 
 ===Handling Duplicated Keys===
 
-Keys within each scope should never be duplicated; all keys in the format are unique. However implementors
-will still need to handle events where keys are duplicated, either duplicated in the transaction
-itself or when combining transactions with duplicated fields. If duplicated keys are
-encountered, the software may choose to use any of the values corresponding to that key.
+Keys within each scope should never be duplicated; all keys in the format are unique. PSBTs containing duplicate keys are invalid. However implementors
+will still need to handle events where keys are duplicated when combining transactions with duplicated fields. In this event, the software may choose
+whichever value it wishes.
 
 ==Responsibilities==