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If you specify either of the options to validate rollback scripts, then you must provide -rollback or _rollback files for all scripts in the project.  The requirement starts when you enable the option. You do not need to provide rollback files for SQL scripts that have already been packaged and deployed. 

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Rollback Files and Developer Workflow Files

Do not use both -rollback/_rollback files and AND also Developer Workflow files (-cleanup/_cleanup, -replace/_replace, -ignore/_ignore) for the same SQL script. In general, Developer Workflow functionality is intended for SQL scripts that do not have -rollback/_rollback files. 

See Developer Workflows: Managing Changesets during Deployment and Test

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