I had a similar experience about a year before Millie cocker came to live with me.
I was walking my foster rescue labrador in a popular local wood, when a gent passed and commented that he'd just seen my double. 'Impossible,' I thought, but then about 100yds further on I bumped into a lady with another yellow lab. We both wanted to know where each other's dog had come from as they were so similar, even down to a special whorl of hair on their tails. Both dogs got on famously. Eventually we worked out that her dog was very possibly one of my dog's pups: they both came from the same town and their ages seem correct.
What was the chamce of that happening? Sheer fluke!
Mum has now gone out of county and is very happy with her new owners.