Ok, this one is a bit odd. In my environment we have FAST Search Server 2010, everything is configured and working properly.
Recently we had to run a full profile sync in the User Profile Service Application to resolve some discrepancies. Since then, when doing a People Search on a FAST Search Center, many people are returning multiple results. What makes it very odd is that one result goes to their NT Login and the other to a GUID.
The one referencing the NT login works as expected. However, when selecting the GUID result, it gives a User Not Found error message.
I had reset the search index on the FAST Query SSA (where the people search results come from) and recrawled. The issue still exists.