Both WiredTigerIndex::BulkBuilder and SortedDataIndexAccessMethod::BulkBuilderImpl are checking for duplicate keys when building indexes. In addition to being redundant dead code, the handling in SDIAMBBI for when WTIBB reports a duplicate is incorrect. Before SERVER-95960 it would be an invariant failure, and after that it bypasses onDuplicateRecord and just reports the error.

Removing the duplicated check will have a small runtime perf benefit and solves a problem where SERVER-89359 would need to start copying the buffer to be able to perform the check.