Which SQL Read TRANSACTION ISOLATION LEVEL do I want for long running insert?

You may want to rethink your process before you use READ UNCOMMITTED. There are many good reasons for isolated transactions. If you use READ UNCOMMITTED you may still get duplicates because there is a chance both of the inserts will check for updates at the same time and both not finding them creating duplicates.

Try breaking it up into smaller batches or issue periodic COMMITS.

The only isolation level that allows one transaction to read changes executed by another transaction in progress (before it commits) is.

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