Both condictio and vindicatio are effective remedies of protecting subjective civil rights. However, the general effectiveness of these remedies is prejudiced by significant legal uncertainty that is due to the fact that the same circumstances can be considered as grounds for bringing both vindicatio and condictio actions at the same time. This situation is traditionally referred to as “competition of lawsuits”. Finding ways to resolve this “competition” is one of the acute issues of modern judicial practice. This problem is not new, though; the lawyers of Ancient Rome faced it. Therefore, it seems reasonable that studying the origins of this problem in the past can help to solve it in the future. In modern Ukrainian academic literature, the raised problem was addressed by I. E. Berestova, I. V. Venediktova, I. M. Honcharov, B. P. Karnaukh, H. V. Puchkova and others. The purpose of the article is to find out the origins of condictio action and to determine the relationship between condictio action and vindicatio action in ancient Roman jurisprudence. First of all the author pays attention to studying the origin of condictio action and underlines that unjust enrichment initially was not the only case where the condictio action was applicable. However eventually ancient Romans realized that condictio action was suitable for returning the unjustified enrichment. Thus the problem arose how to reconcile condictio and vindicatio. In the Roman law of the Justinian Codification period, the idea was established that the condictio and vindicatio suits are in relation of electoral competition, which means that the injured party has the right to choose, at his own discretion, which claim to bring. However, many fragments from Digest show that in the classical period, the views on this problem were fundamentally different, and the choice between condictio and vindicatio was not allowed in principle. According to the views of classical lawyers, the vindicatio claim had to be made when the injured party, despite the loss of possession, remained the owner of the thing; on the contrary, if the injured party, without proper legal grounds, has lost the title of property (for example, because of mixing things with the defendant's assets, consuming or processing them) then in the only appropriate remedy is condictio action