My response to this poll is an
“it depends” one, given that Toowoomba is a small city and is so far off the beaten track, that Leonard is never likely to visit it by chance!
So, I would handle the situation as follows:
a)
If it was around Sept 21st, and the particular date when we have our annual birthday celebration event (which in recent years he has routinely been invited to, through his management’s contact email), I would go up to him and say, “Mr Cohen, fantastic! You’ve made it – how wonderful to surprise us like this!”

The conversation would then be centred on making him feel welcome, comfortable and relaxed about his visit, with reassurances that he would not be overwhelmed at our relatively small gathering in this out of the way place (where few would even recognise him).
a)
If it was any other time of the year, I think I would have to assume it was a double of Leonard’s and so I would say, “Excuse me sir and forgive me for being so bold, but you are the spitting image of Leonard Cohen!”

I would be fairly confident in saying this, though I would be secretly hoping for the following Canadian accented reply, accompanied by a wry smile: “Yes sir and I do believe that cannot but be the case!” I trust the conversation would just naturally take off from there!
Cheers
Andrew (Darby)