You would have to build separate lists, then random sample them.
As Steven and Courtland mentioned, cleaning up dupes first, then creating the desired Smart Lists to apply Random Sample should help reduce the possibility of a email clash.
Or maybe what you need to do is completely separate out the geos in a rough A/B with no random sample. Then you are assured of what Courtland suggests, at least that the same people at the same office location won't get two different emails.