Using this validation method in conjunction with ActiveRecord::Base#save does not guarantee the absence of duplicate record insertions, because uniqueness checks on the application level are inherently prone to race conditions.
And the docs also offer some solutions:
This could even happen if you use transactions with the ‘serializable’ isolation level. There are several ways to get around this problem:
By locking the database table before validating, and unlocking it after saving. However, table locking is very expensive, and thus not recommended.
By locking a lock file before validating, and unlocking it after saving. This does not work if you‘ve scaled your Rails application across multiple web servers (because they cannot share lock files, or cannot do that efficiently), and thus not recommended.
Creating a unique index on the field, by using ActiveRecord::ConnectionAdapters::SchemaStatements#add_index. In the rare case that a race condition occurs, the database will guarantee the field‘s uniqueness.
This typically isn't something you'd need to worry about until you get to some traffic of scale and size. So don't worry about it too much until you get there, but be aware of the problem. Read the docs for more details and information. tip!