One reason an append query can fail is because of duplicate primary keys in the source and destination tables.

Name: One reason an append query can fail is because of duplicate primary keys in the source and destination tables.
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An append query adds new data from the source table to the destination table. If there are duplicate primary keys in both tables, the query will not be able to uniquely identify where to add the new data and it will fail.

One Reason an Append Query Can Fail Is Because of Duplicate