Multiple documents with the same _id value make reIndex invariant

In a cluster affected by WT-7995 and WT-8395, there two documents sharing the same _id value.

Our remediation process guide requires running reIndex to fix inconsistencies detected by validate. In master, when reIndex encounters duplicate _ids it invariants. The server ends up in a bad state because it tries to build the _id index during recovery but fails every time due to the duplicate values.

In 4.4 the behaviour is a bit different, reIndex adds the duplicate entries to the index and validate invariants when it encounters two duplicate values in a unique index.