- Same OS, server: you likely control all the processes in it anyway, no untrusted code.
- Different OSes in different VMs, server: they likely run different versions of Linux kernel, libc, and shared libraries anyway. They don't share the page caches for code they load from disk.
- Browser with multiple tabs, consumer device: likely they might share common JS library code on the browser cache level. The untrusted code must not be native anyway, and won't load native shared libraries from disk.
- Running untrusted native code on a consumer device: well, all bets are off if you run it under your own account; loading another copy of msvcrt.dll code is inconsequential compared to the threat of it being a trojan or a virus. If you fire up a VM, see above.
In any case, containers share OS kernel, OS page cache, etc. This can be beneficial even for a shared hosting as a way to offer a wide range of preinstalled software as ro-mounted into the container's file tree. Likely code pages of software started this way would also be shared.