What Is the Law That Allows a Person to Sue Only Once For the Same Issue?
A Lawsuit or Other Legal Proceedings That Dispute a Specific Issue May Be Brought Only Once By the Plaintiff Against the Same Defendant. Bringing the Same Lawsuit More Than Once Is Forbidden As Per the Issue Estoppel Principle Which Is Also Known As Res Judicata Which Is Latin For 'Things Decided'.
A Helpful Guide For How to Determine and Understand Whether Going to Court Is Actually Going Back to Court?
The res judicata principle, which loosely means 'things decided' in Latin, also known as issue estoppel, can be confusing to both laypeople and lawpeople. The legal principle of res Judicata is used to as a barrier to further judicial proceedings on a particular issue whereas, for various reasons, the judicial process requires and expects that litigation or other legal proceedings will achieve finality. One reason for final conclusion of legal proceedings is that the administration of justice, meaning the justice system itself, may fall into disrepute in the public eye if parties to proceedings were able to relitigate matters over and over again. Additionally, conducting legal proceedings over and over again would be a waste of taxpayer funded judicial resources.
Whether a legal case is subject to the res judicata principle is such a commonly raised issue that the Supreme Court precedent decision of Danyluk v. Ainsworth Technologies Inc.,  2 S.C.R. 460 that addresses the issue is cited over 1,800 times upon the CanLII.org website. As for legal test of what is deemed a legal case barred by the res judicata principle, such was stated by the Supreme Court in Danyluk where it was said:
When a question is litigated, the judgment of the Court is a final determination as between the parties and their privies. Any right, question, or fact distinctly put in issue and directly determined by a Court of competent jurisdiction as a ground of recovery, or as an answer to a claim set up, cannot be re-tried in a subsequent suit between the same parties or their privies, though for a different cause of action. The right, question, or fact, once determined, must, as between them, be taken to be conclusively established so long as the judgment remains. [Emphasis added.]
This statement was adopted by Laskin J. (later C.J.), dissenting in Angle, supra, at pp. 267-68. This description of the issues subject to estoppel (“[a]ny right, question or fact distinctly put in issue and directly determined”) is more stringent than the formulation in some of the older cases for cause of action estoppel (e.g., “all matters which were, or might properly have been, brought into litigation”, Farwell, supra, at p. 558). Dickson J. (later C.J.), speaking for the majority in Angle, supra, at p. 255, subscribed to the more stringent definition for the purpose of issue estoppel. “It will not suffice” he said, “if the question arose collaterally or incidentally in the earlier proceedings or is one which must be inferred by argument from the judgment.” The question out of which the estoppel is said to arise must have been “fundamental to the decision arrived at” in the earlier proceeding. In other words, as discussed below, the estoppel extends to the material facts and the conclusions of law or of mixed fact and law (“the questions”) that were necessarily (even if not explicitly) determined in the earlier proceedings.
25 The preconditions to the operation of issue estoppel were set out by Dickson J. in Angle, supra, at p. 254:
(1) that the same question has been decided;
(2) that the judicial decision which is said to create the estoppel was final; and,
(3) that the parties to the judicial decision or their privies were the same persons as the parties to the proceedings in which the estoppel is raised or their privies.
The issue estoppel principle along with the requisite three elements was well summarized within the case of Roumanes v. Dalron et al., 2010 ONSC 2891 wherein it was said:
 In the case of Danyluk v. Ainsworth Technologies Inc.  2 S.C.R. No. 460, the Supreme Court of Canada determined that the application of issue estoppel requires a balancing of the public interest in the finality of litigation with the public interest in ensuring that justice is done on the facts of a particular case. Its application involves a two step process: firstly, the determination of whether the three preconditions of issue estoppel have been met, and secondly, if they have been met, the determination of whether it ought to apply in the particulars circumstances of the case.
 The three preconditions of issue estoppel were stated as follows: (1) that the same question has been decided; (2) that the judicial decision which is said to create the estoppel was final; and (3) that the parties to the judicial decision or their privies were the same persons as the parties to the proceedings in which the estoppel is raised.
Accordingly, the test of whether issue estoppel exists and whether a matter should be heard essentially comes down to three simple factors, all of which must met, and being whether:
- The same legal question was previously decided;
- The judicial decision was a final decision; and
- The parties to that decision were the same parties as within the new proceeding.
Accordingly, if the previous decision addressed a different question of fact or law, if the previous decision was without finality, or if the previous decision involved different parties to the proceeding, the fresh proceeding is other than subject to the res judicata doctrine and should proceed as a separate matter, even if stemming from - and perhaps especially as stemming from, the previous matter.