It depends on whether or not I recognize them, and even if I do, it depends on if I can remember their name, too.
I suffer from facial amnesia, so I usually have to have seen people a number of times before their names make a dent in my memory banks. Growing up, I didn't see much of my mother's side of the family because they lived in other states. I once attended a funeral for one of my adult maternal cousins. I extended my sympathies to the couple that I thought were his parents. They were a different aunt and uncle.
Fortunately, they didn't realize my mistake because they had had care of him for a while when he was a child, so it wasn't really all that inappropriate. I did eventually find the right aunt and uncle to extend sympathies to.
When I am out shopping people occasionally come up to me who I have apparently met before, and greet me and start talking to me, and I'm racking my brain, thinking, "Who are these people?" It's very awkward when that happens. I do my best to be friendly though, while hoping that their side of the conversation provides clues as to their identity, and how I am supposed to know them. 
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau