duplicate insert into namespace_statistics during v2 migration

[aryairani]

Describe and demonstrate the bug

$ git clone https://github.com/DataKinds/unison bug
$ ucm -c bug

📋 I backed up your codebase to bug/bug/.unison/v2/unison.sqlite3.v1.1719713918
⚠️  Please close all other ucm processes and wait for the migration to complete before interacting with your codebase.
Press <enter> to start the migration once all other ucm processes are shutdown...

🔨 Migrating codebase to version 2...
Starting codebase migration. This may take a while, it's a good time to make some tea ☕️
Updating Namespace Root...
 🏗  138 / ~6577 entities migrated. 🚧Uh oh, an unexpected exception brought the process down! That should never happen. Please file a bug report.

Here's a stringy rendering of the exception:

  SqliteQueryException {sql = "INSERT INTO namespace_statistics ( namespace_hash_id, num_contained_terms, num_contained_types, num_contained_patches ) VALUES (?, ?, ?, ?)", params = [SQLInteger 6776,SQLInteger 0,SQLInteger 0,SQLInteger 0], exception = SQLite3 returned ErrorConstraint while attempting to perform step: UNIQUE constraint failed: namespace_statistics.namespace_hash_id, callStack = [], connection = Connection { name = "main", file = "bug/.unison/v2/unison.sqlite3" }, threadId = ThreadId 12}

Environment (please complete the following information):
v0.5.23

Additional context
blocks my attempted repro of #2859