My friend and I have matching house keys

I had known my friend for several years. She lived about a mile from me and during that time we had frequently swapped keys so we could feed one another's pets or water the plants. For a couple of years I even had a spare key for her which I kept in a dish in my hall. It was attached to a key fob which had a photograph of her daughter on it, and this meant I could always identify it as my friend's key rather than one of ours. One day I noticed my husband, who was unable to find his own key, had used this spare key to lock our front door! He had not noticed the picture on the key fob. I was very surprised by this, and thought there must have been some mistake, as the key worked perfectly. I rang my friend, and arranged that she should try to unlock my front door with her own door key! Sure enough, we confirmed that the our keys operate each other's front door locks perfectly! Her lock was the same one that had been there when she bought the house 13 years earlier. We had replaced our front door, and lock, some six years earlier. I have never met anyone else who has found their keys match in this way, although I realise there must be a finite number of patterns for keys. We find it quite handy, as once or twice when we have been locked out of our house, my friend has come to our rescue! However, we do joke that if our friendship ever ended we would both have to change our locks!