From Carr v. Tillery, 2010 U.S. App. LEXIS 685 (7th Cir. Jan. 12, 2010):

Rex Carr, a successful class action lawyer in southern Illinois, is locked in mortal combat with his former law partners, the defendants in a RICO case (with a supplemental state-law claim, 28 U.S.C. § 1367) that he brought in federal district court. The dispute is over the division of legal fees in cases handled by the law firm (Carr Korein Tillery, LLC) before it broke up; Carr is seeking some $20 million in compensatory damages alone. The district court dismissed the entire case, supplemental claim and all, under Rule 12(b)(6) (failure to state a claim), on the ground that Carr's claims are precluded by judgments in previous suits by him against the same defendants. Since res judicata is an affirmative defense, the defendant should raise it and then move for judgment on the pleadings under Rule 12(c). Forty One News, Inc. v. County of Lake, 491 F.3d 662, 664 (7th Cir. 2007); McCready v. eBay, Inc., 453 F.3d 882, 892 n. 2 (7th Cir. 2006). The judge thus jumped the gun in dismissing the case under Rule 12(b)(6). But the error is of no consequence. He had before him all he needed in order to be able to rule on the defense, and anyway the plaintiff does not complain about the error. ***

The complaint in his present suit repeats many of the charges in the 2004 suits, including the charge that he was fraudulently induced to sign the Memorandum of Understanding and that the defendants had violated the previous fee-allocation agreements, which the Memorandum had superseded. All those charges are barred by the dismissal with prejudice of the 2004 suits. The fact that the present suit redescribes the wrongful acts alleged in the earlier ones as predicate acts in support of the RICO claim is irrelevant. You cannot maintain a suit, arising from the same transaction or events underlying a previous suit, simply by a change of legal theory. That is called "claim splitting," and is barred by the doctrine of res judicata. Nowak v. St. Rita High School, 197 Ill. 2d 381, 757 N.E.2d 471, 478-79, 258 Ill. Dec. 782 (Ill. 2001); River Park, Inc. v. City of Highland Park, 184 Ill. 2d 290, 703 N.E.2d 883, 893, 234 Ill. Dec. 783 (Ill. 1998); Curtis v. Lofy, 394 Ill. App. 3d 170, 914 N.E.2d 248, 258-59, 333 Ill. Dec. 41 (Ill. App. 2009); Brzostowski v. Laidlaw Waste Systems, Inc., 49 F.3d 337, 338-39 (7th Cir. 1995) (Illinois law).

As if this were not enough, those charges (and more, as we'll see) are also barred by Illinois's "one refiling," or, as it is sometimes called, "single refiling," rule. In March and April 2007, while the dismissal of Carr's counterclaim in the earlier litigation was pending on appeal, he filed four lawsuits against the defendants in Illinois state courts. He filed the present suit three weeks after the filing of the fourth state-law suit, and within days of doing so voluntarily dismissed all four suits; the dismissal orders state that the dismissals are without prejudice. Illinois law provides that a plaintiff who voluntarily dismisses a suit "may commence a new action within one year or within the remaining period of limitation, whichever is greater." 735 ILCS 5/13-217. The Illinois courts interpret this to mean that a plaintiff who voluntarily dismisses a suit may commence only one new action.