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'.
Understanding When Going to Court Is Improperly Going Back to Court Contrary to the Res Judicata Principle
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.
The question of whether a matter is subject to the principle of res judicata arises so frequently that the Supreme Court precedent on the subject, Danyluk v. Ainsworth Technologies Inc.,  2 S.C.R. 460, is cited more than 1,800 times on CanLII.org. The criteria for what constitutes as a matter barred by the principle of res judicata was stated by the Supreme Court in Danyluk:
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.
Interestingly, a key requirement for the principle of issue estoppel, or res judicata to come into play requires that a judicial decision on the merits of the issue were made. This point is sometimes lost among legal professionals and sometimes even judges, who fail to fully appreciate and recognize the element of prior "judicial decision". Accordingly, where parties to a dispute agree to settle, agree to discontinue, agree to dismiss, or otherwise terminate a legal proceeding prior to a judicial decision, the issue estoppel or res judicata doctrine is unable to apply to any subsequent proceeding involving the same issues. Of course, this is said without saying that a subsequent proceeding is without bar, such as expiry of a limitation period, or other legal principles; however, the principle of issue estoppel fails to apply to a legal proceeding commenced where the prior legal proceeding ended prior to adjudication on the merits. This was well explained in the case of Rintoul v. Drummond, 2022 ONSC 998 wherein it was said:
 The defendants argue that the dismissal of the plaintiffs’ claim represented a with prejudice disposition of the plaintiffs’ claims and that the legal doctrine of res judicata precludes them from being re-litigated in this action. Mr. Drummond asserts that he was led to understand that the dismissal of the Small Claims Court proceeding would put an end to legal proceedings arising from use of the OWB once and for all. The defendants rely on the decision of Ground J. in Reddy v. Oshawa Flying Club (1992), 11 C.P.C. (3d) 154 (Ont. Gen. Div.), at para. 9:
The case law seems to be clear that a consent order which ends an action is of the same effect for purposes of the res judicata doctrine as a judgment issued by the court on completion of a trial or hearing.
 The plaintiffs point out that the co-defendant in this action, Ms. Capello, was not a party to the Small Claims Court proceeding. But that aside, it was clearly the plaintiffs’ intention to discontinue their action so that they could start a new one in the right court.
 In my view the application of a bar based or res judicata would not be appropriate in this case. A discontinued proceeding would not operate as a bar. And even if the operative termination of the Small Claims Court action was by a consent dismissal order signed by the Registrar, it was clearly not the intention of the plaintiffs to foreclose their ability to bring an action in this court.
 Furthermore, there cannot be said to have been an adjudicative aspect to the consent order. In Lawyers' Professional Indemnity Co. v. Geto Investments Ltd. (2001), 2001 CanLII 27980 (ON SC), 54 O.R. (3d) 795 (S.C.J.), commenting on the above quoted passage from Reddy, Nordheimer J. wrote, at paras. 11 and 12:
 With respect, I do not believe that the authorities support the breadth of that statement. Where the consent order has an adjudicative aspect to it, as it did in Maiocco v. Lefneski,  O.J. No. 4014 (Ont. Gen. Div.)] and in Staff Builders [International Inc. v. Cohen,  O.J. No. 401 (Ont. H.C.)], then the principle of res judicata may apply. But where, as here, the order does nothing more than dismiss the action pursuant to a settlement and therefore clearly does not purport to have made any adjudication of the action on the merits, then I do not see how the doctrine of res judicata can flow from it. The classic statement in Henderson v. Henderson (1843), 3 Hare 100, (Eng. V.-C), which is relied upon both in Reddy v. Oshawa Flying Club, supra, and in Staff Builders International Inc. v. Cohen, supra, says, at p. 115:
The plea of res judicata applies, except in special cases, not only to points upon which the Court was actually required by the parties to form an opinion and pronounce a judgment, but to every point which properly belonged to the subject of litigation, and which the parties, exercising reasonable diligence, might have brought forward at the time. (emphasis added)
 As I have already indicated, the consent order here does not involve a pronouncement of judgment on any of the matters raised in the former action nor could it given that the order itself was granted by the Registrar. In my view, therefore, the doctrine of res judicata is not invoked by such an order and no estoppel can arise as a consequence of it.
Simply said, unless a prior proceeding was adjudicated upon at Trial or some type of judicial decision at a Motion where the decision was based upon substantive merits rather than even procedural concerns, the issue estoppel doctrine fails to attach and is unavailable to bar a subsequent 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.