This is just a common process of the launcher, similar to how Chrome, Discord, and Steam function. This allows the programs to multitask more efficiently and, in the case of Chrome, preserve backups in case of a program crash. They are designed to work like this and it is intentional.
The only time this is an issue is if you see multiple Maplestory.exe processes (the one with the mushroom icon), in which case you can simply select and end the processes in the task manager.
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The only time this is an issue is if you see multiple Maplestory.exe processes (the one with the mushroom icon), in which case you can simply select and end the processes in the task manager.