DOCSP-37071 Duplicate Key Error FAQ (#389) (#400)

* DOCSP-37071 Duplicate Key Error FAQ

* JD feedback

Author: ajhuh-mdb

source/faq.txt

@@ -140,6 +140,28 @@ These logs do not indicate that ``mongosync`` is failing or corrupting
 data. If a fatal error occurs, ``mongosync`` stops the sync and writes a
 fatal log entry. 
 
+What if I see a lot of duplicate key errors in the logs?
+--------------------------------------------------------
+
+Duplicate key errors are a normal part of the sync process. These errors can 
+occur if:
+
+- You insert a document on the source cluster after ``mongosync`` starts. 
+  ``mongosync`` may directly copy the document and redundantly apply the insert 
+  change event for the document later.
+- You stop and resume ``mongosync``. This can lead to duplicate insertions when 
+  ``mongosync`` restarts.
+- ``mongosync`` encounters a transient error and reattempts an insertion that 
+  may have already succeeded.
+
+What should I do if mongosync returns a fatal error?
+----------------------------------------------------
+
+A fatal error indicates a problem that must be corrected and requires the 
+migration to be restarted. After addressing the error, delete all migrated 
+data on the destination cluster, including the ``mongosync_reserved_for_internal_use`` 
+database. Then, relaunch ``mongosync`` and start a new migration.
+
 Does ``mongosync`` Support TTL Indexes?
 ----------------------------------------