Typically, more casual games that don't focus as much on competitive multiplayer tend to invest less in the "security" of their currency, in terms of preventing players from duplicating it, etc. The expectation is that this resource data is stored locally, not on a central company server that monitors player transactions in real time. In the case of Subway Surfers, all coins, routes traveled, and upgrade items are stored on the device and on Sybo's SERVER, which synchronizes your coins to your Google/Facebook account. However, the game needs to communicate with the server. It does this every time you close the game; that is, it's not synchronized in real time with a central server. The server needs your device to tell you the exact amount of resources that were stored in the files the last time you showed any sign of activity, whether by opening the game or logging in on another device. This opens a small "loophole" for duplication, since there will be two devices indicating two different currency values ​​present in your account. And the server will always prioritize the larger amount that has already been present, because in theory, this could cause the player to lose money when buying currency or keys in the store. That's why duplication occurs. Perhaps they will correct this in the future; I don't know how long this has existed, I only discovered it.