In my early twenties I was in a relationship with a 6' 5" Scotsman, who I lived with for a few years. Around 3 years after we had broken up, my best friend (who had also known the boyfriend well) and I happened to spot him doing some Christmas shopping in town.
It was a busy city centre shopping area, so despite being only a few metres away from him, I called his name quite loudly, several times. I felt a little miffed that he was either ignoring me, or couldn't hear me calling him, but having parted on good terms and having been his "other half" for more than 3 years, I had no qualms about walking up to him and throwing my arms around his middle (I'm a shortarse who only ever reached his mid-chest) whilst exclaiming (fondly!); "It's great to see you, Sean, you miserable old bastard!"
I felt his body stiffen (no, not like that...), and he placed his hands firmly on my shoulders and pushed me away from himself, whilst he looked down at me with his face an expression of shock and total confusion.
"Can I help you?" he asked. "Errr, hello? It's me - Betsy..." I replied. My brain was starting to register something odd. "I'm sorry..." he started, then I realised - he had completely lost all trace of his very strong Scottish accent!
"What's happened to your voice, Sean?" I asked, as my best friend continued to look on in confusion.
"My name is Jonathan," came his response. 
"Oh. Right. Sorry then. Thought you were someone else." I stuttered as I grabbed for my friend and she carefully steered me away in the opposite direction! 
I was absolutely mortified - I had been utterly convinced that he was the man I'd slept next to each night for more than 3 years, when in reality, he was a complete stranger! He looked SO like my ex, even when I had been up really close to his face
that in spite of the humiliation still heating my entire face, there was still a part of my brain which refused to be convinced and was still thinking he was just teasing me (thank God for my friend managing to 'talk me down')! I remember sitting having a strong coffee in a (not too) nearby café afterwards, and eventually concluding to my friend: “I should have known, he smelled totally different to Sean…” Oh God. 
I had never really considered doppelgangers until then, but his man truly was a perfect match for the “original”! I still go hot all over when I think of me hugging that mountain of stranger, and of course the fact that after having done that, I then called him by the wrong name and walked away without giving him even a simple explanation!
Arrrgh, I’m going to keep having the horrors each time the memory waves at the edges of my consciousness for the next few days!