I dont think it all that uncommon, specialy as the keys start to wear. When I was going to BCIT I had a 91 S10 and a guy in my class had the exact same year/ make/ model. My keys fit in his door and his ignition but his would not open or start my truck. One day after class he stayed late to finish a project so I drove his truck to the other end of the lot and left it there, he was smart enough to know I did it and didnt think it was stolen so he meandered around the parking lot for an hour till he found it, He was sure pissed at me the next day.
I am sure some kid in my neighborhood has a key that will unlock the door on my 2000 GMC. I know I lock it (horn sounds when it locks and I always double check it) but twice now I have come out in the morning to find my drivers door open with absolutely no damage, first time they took my CDs now I leave absolutley nothing in it.
I also recently learned that all travel trailers/ 5th wheels and campers have the same key for the storage doors. When I asked our sale guy about it he couldnt give me an answer other than for simplicity and cost. Whats the point of having a lock?