Nixpkgs doesn't wontonly package multiple versions of libraries. But as you upgrade you will accumulate old builds of libraries until you GC. The meaningful question is not how many of X do you have installed, but whether you have multiple versions of X reachable from GC roots.
- Traditional distro: you cannot end up with that
- Flatpack, snap etc.: you cannot avoid that
Nixpkgs doesn't wontonly package multiple versions of libraries. But as you upgrade you will accumulate old builds of libraries until you GC. The meaningful question is not how many of X do you have installed, but whether you have multiple versions of X reachable from GC roots.