A more thorough explanation:

In civil procedure, multiplicity of actions refers to the situation where multiple lawsuits are brought up that raise the same issue(s) and could have been brought up in one action. This is generally avoided because it can lead to inconsistent outcomes and is a waste of public resources.

• Compulsory Counterclaims: Rule 13(a) of the Federal Rules of Civil Procedure (FRCP) establishes the federal practice of compulsory counterclaims. This means that if a defendant has a claim against the plaintiff that arises from the same transaction or occurrence as the plaintiff's claim, they must bring it up in the same lawsuit. This avoids the possibility of the defendant bringing up a separate lawsuit later on, which would be a waste of resources and could lead to inconsistent outcomes.
• Third-Party Impleader: FRCP 14 establishes the third-party impleader, which allows a defendant to bring in a third party who may be liable for some or all of the plaintiff's claim. This avoids the need for the plaintiff to bring up a separate lawsuit against the third party, which would be a waste of resources and could lead to inconsistent outcomes.
• Mandatory Joinder of Parties: Courts may apply mandatory joinder of parties to avoid the undesirability of duplicative litigation. This means that if there are multiple parties who are necessary for a complete resolution of the dispute, they must all be included in the same lawsuit. This avoids the possibility of separate lawsuits being brought up later on, which would be a waste of resources and could lead to inconsistent outcomes.

These examples illustrate how the law seeks to avoid multiplicity of actions by requiring related claims to be brought up in the same lawsuit and by joining all necessary parties in the same lawsuit. This avoids the waste of resources and the possibility of inconsistent outcomes that can result from multiple lawsuits on the same issue(s).