When could 2 virtual addresses map to the same physical address?

Question

An operating system/computer architecture question here. I was reading about caches, about how virtually indexing the cache is an option to reduce address translation time. I came across the following:

"Virtual cache difficulties include:
    Aliasing
        Two different virtual addresses may have the same physical address."

I can't think of a scenario when this can occur. It's been a while since my O/S days and I'm drawing a blank.

Could someone provide an example? Thanks

Answer 1

Two processes might have a shared mapping. E.g., in Unix, executable code is typically mapped into a region shared between all processes that execute the same program. (In fact, a single process might have several mappings of the same underlying memory, e.g. when it mmap's the same file twice.)

Answer 2

I believe that the executable sections of programs can possibly be shared between processes--thus being mapped twice.

For example: if you load two instances of vim, there will be two processes. Both process will likely map to the same executable code in physical memory.