A more thorough explanation:

Definition: The mutuality doctrine is a legal principle that requires both parties to have been in privity with one another in an earlier proceeding for collateral-estoppel to bar a party from relitigating an issue determined against that party in the earlier action. It is also known as mutuality of parties.

Examples: If a person sues a company for breach of contract and loses, they cannot sue the same company again for the same issue unless both parties were in privity with one another in the earlier proceeding. For instance, if the person was an employee of the company during the first lawsuit, they cannot sue the company again as a customer or vendor. This is because the mutuality doctrine requires both parties to have been in privity with one another in the earlier proceeding.

Another example is if a landlord sues a tenant for unpaid rent and wins, the tenant cannot sue the landlord for the same issue unless both parties were in privity with one another in the earlier proceeding. For instance, if the tenant sublets the property to another person during the first lawsuit, they cannot sue the landlord again for the same issue. This is because the mutuality doctrine requires both parties to have been in privity with one another in the earlier proceeding.

These examples illustrate how the mutuality doctrine works in practice. It prevents parties from relitigating the same issue repeatedly and ensures that both parties have a fair chance to present their case in court.