Question for locksmiths, et al

[JasonGraham]

Recently replaced an appliance for a tenant, and while there, was told that her boyfriend's house key fits all of her locks as well. Her boyfriend was there and showed me. The teeth pattern on his key looks very similar to hers, with very slight differences.

Anyone heard of this before? For reference, his house is about 25 mile away. I had the entire house re-keyed when she moved in about 5 months ago, and will rekey it again, but to me, this is lotto odds.
Am I wrong? Are they scamming? I have no idea to what end, they both said it was no big deal, just strange..... I have a call into the locksmith who did the work, and will see what he says.
Possibly our TGP blues locksmiths have thoughts as well.

[Kalisspera]

I see it's been a while since you’ve posted, but I just wanted to ask—did you ever find a solution? I've run into a similar thing with old emergency exit hardware, and was wondering if any locksmiths here have tips on working with aged or obsolete parts. Do manufacturers still support these or is it mostly a hunt through surplus and custom fixes? Curious to hear more experiences.

[MikeBrooklyn]

That is weird, but not impossible—I've had a tenant once whose old mailbox key opened her new flat’s front door. Total fluke, but it happened. Turns out some cheap lock cylinders can have overlapping key patterns if cut a certain way. You’re smart to rekey again, especially for peace of mind. I’d also double-check with local locksmith near you—I’ve used Emergency Locksmiths in London before when locked out. Fast, honest, and explained way more than I expected.