A more thorough explanation:

The first-filing rule, also known as the first-to-file rule, is a principle in civil procedure that states that when two parties file lawsuits regarding the same issues in two courts of proper jurisdiction, the court that first acquires jurisdiction usually retains the suit, to the exclusion of the other court.

For example, if a plaintiff files a lawsuit in a state court and the defendant files a lawsuit in a federal court regarding the same issue, the court that first acquires jurisdiction will usually retain the case, and the other court will stay proceedings or abstain.

However, if the first-filed suit is brought merely in anticipation of the true plaintiff's suit and amounts to an improper attempt at forum-shopping, an exception exists, and the second court may proceed with the case.

Another aspect of the first-filing rule is the doctrine allowing a party to a previously filed lawsuit to enjoin another from pursuing a later-filed action.

For example, if a plaintiff files a lawsuit against a defendant, and the defendant files a similar lawsuit against the plaintiff in another court, the plaintiff may seek an injunction to prevent the defendant from pursuing the later-filed action.

Overall, the first-filing rule is designed to prevent forum-shopping and promote judicial efficiency by avoiding conflicting judgments and duplicative litigation.