Why SingleOrDefault Can Be a Hidden Data Integrity Risk in Your Codebase

Using SingleOrDefault or SingleOrDefaultAsync in database queries acts as an implicit assertion that only zero or one matching row exists, but the method itself does not enforce uniqueness at the storage level. If duplicate rows are present, the query throws an exception that can disrupt normal request handling before validation or authorization even runs. A developer reviewing a real-world recovery case found that the key challenge was containing the failure safely without masking the underlying data defect. The recommended approach separates two goals: short-term containment through a deterministic fallback query, and long-term correction by repairing duplicate data and adding proper database constraints. Critically, any resolved candidate must still pass all security and authorization checks, as containment should stabilize the lookup path rather than weaken downstream boundaries.