Your subject line says that the apps are downloading from the Play Store, but your actual post just says that the apps appear there. Are they actually being downloaded from the Play Store, or do they just appear in the list of installed apps in the Play Store?
I have apps which could be installed from either F-Droid or the Play Store, and in these cases apps that I installed from F-Droid also appear in my installed apps list in the Play Store (and vice-versa). And this does make sense, because you don't want to download an app from one store when you've already installed it from another. It's a touch annoying that there is nothing in the Play Store listing that tells you that it wasn't installed from the Play Store (you have to go into Settings > Apps, find the app and look in the "advanced" information to see where it was installed from), but in fact each app is only updated by the store that installed it, you don't have second copies installed or the two stores fighting to get their version in as the next update.
So my guess is that this is the same thing you are seeing: you install an app from the Samsung store, the same app exists in the Play Store catalogue, so the Play Store shows the app as installed. But unless you start seeing both stores offering updates to the same app this is normal, does not mean that the Play Store is downloading copies of apps you install from other places, and doesn't require any action to correct it.