Ввод rails generate model --help
даст вам нужный ответ:
Runtime options:
-f, [--force] # Overwrite files that already exist
-p, [--pretend] # Run but do not make any changes
-q, [--quiet] # Supress status output
-s, [--skip] # Skip files that already exist
В последней части вы видите:
Skip files that already exist