If you mean by accident, all the time mate!!! Especially with me working away from home a lot, I can't always remember what I've got.
Eg fairly recently, American Youth Choir UK Polydor, Betty Everett, I've Got A Claim On You, UK Sue, 3 copies . . . seriously. Think one might be a Demo?
If by design then again, yes. For instance, Scott Bros - Stolen Angel - on Ribbon issue and Demo, local label called Dorset, and on the Canadian label Castle. Also came out on Cameo (or Parkway) but it's a different / inferior cut.
Other occasions are when I get issues and Demos. Can never decide which one to keep? Eg Patrice Holloway, UK Capitol, Break Out, Mitch Ryder, Stateside, (as well as on Soul Sounds too). So, I keep both, especially if they're both in the same condition. It's a curse.