Next is the problem of duplicate suits. Where the debtor has already filed suit against the third-party debtor and the collecting creditor files a collection suit against the same third-party debtor, whether it is a duplicate suit is disputed. The Supreme Court held that, because the debtor's suit becomes improper under the collection order, the collection suit does not constitute a duplicate suit. Since it is a special relationship in which the debtor loses the authority to conduct the litigation, a suit filed by the collecting creditor after the suit filed by the debtor is not a duplicate suit.

Finally, there is the problem of joint litigation and intervention. Where several collecting creditors file collection suits against the third-party debtor over the same seized claim, or another collecting creditor intervenes in an existing suit, the question is how to define their litigation relationship. Article 249(2) of the Civil Execution Act expressly provides that all creditors may join the plaintiff's side as co-litigants, allowing all creditors to participate in the collection suit. This provision presupposes that the effects of the judgment, such as res judicata, will extend to the participants.

The case law held that the res judicata of a collection suit does not extend to other creditors who received collection orders before the close of pleadings, but where several collecting creditors jointly file suit over the same seized claim, it is reasonable to regard it as quasi-compulsory joint litigation. This is an interpretation consistent with Article 249 of the Civil Execution Act.

The difference between collection suits and creditor's subrogation suits is where the Civil Execution Act and the Civil Act meet, so it frequently appears on exams and assignments. Reading Article 218 of the Civil Procedure Act, which sets the subjective scope of res judicata, together with Article 249 of the Civil Execution Act, which provides for collection suits and orders to participate, makes it clearer why the case law used the close of pleadings as the reference point, and lets you build arguments for and against in a balanced way in a case-comment assignment. In particular, the distinction between substitutive and parallel litigation on another's behalf connects directly to the issue of standing, so studying it together with the unit on parties in civil procedure deepens your understanding considerably.