ActiveRecord::RecordInvalid: Validation failed: Email has already been taken (ActiveRecord::RecordInvalid)

## Error

```
Validation failed: Email has already been taken (ActiveRecord::RecordInvalid)
```

## Stack trace

```
active_record/validations.rb:84 in raise_validation_error
active_record/validations.rb:55 in save!
active_record/transactions.rb:313 in block in ActiveRecord::Transactions#save!
active_record/transactions.rb:365 in block in ActiveRecord::Transactions#with_transaction_returning_status
active_record/connection_adapters/abstract/transaction.rb:535 in block in ActiveRecord::ConnectionAdapters::TransactionManager#within_new_transaction
active_support/concurrency/null_lock.rb:9 in synchronize
active_record/connection_adapters/abstract/transaction.rb:532 in within_new_transaction
active_record/connection_adapters/abstract/database_statements.rb:344 in transaction
active_record/transactions.rb:361 in with_transaction_returning_status
active_record/transactions.rb:313 in save!
```

## Tracking Details
- **Project**: exercism-rails
- **Sentry**: [ActiveRecord::RecordInvalid: Validation failed: Email has already been taken (ActiveRecord::RecordInvalid)](https://thalamus-ai.sentry.io/issues/98500518/)
- **Culprit**: Auth::OmniauthCallbacksController#github
- **First seen:** Feb 23, 2026