Too many instances of an app in recent apps menu

[AC Question]

I don't get it, everytime I open this one app by tapping its pushup notification it creates a new instance of itself in the recent apps menu, so when I tap on the recent apps button on my phone there are two instances of the same app, if I open it again by tapping on its notification it creates another instance, and so on. It's really annoying, can anyone tell me why that's happening? I don't know if it's happening only with that app, because I don't have other apps that use pushup notifications.

Samsung Galaxy s5 g900f, Android 6.0.1 TW