I have a whole box of duplicates, copies of CDs that I've repurchased in RVG form, new reissues, etc.
I frequently give these away to students/friends, if it's something they want or something I want them to hear. When I started out with jazz, friends gave/lent me stuff to listen to, and that meant a lot to me. I feel like I'm continuing a tradition, like it's good karma. Nothing quite like giving a young jazz fan their first copy of KOB (or whatever).
If someone with whom I have a reasonable level of trust asks to borrow something, I will let them. If they keep it or lose it, that's just life. No big deal, it's just "stuff". If I ever borrow something I assume total responsibility for it- I've replaced cracked jewel cases etc. I don't borrow very much- I can barely keep up with my own purchases!
That being said, some of the more valuable items don't leave the house- Mosiacs, rare items etc. I don't even like taking the Mosaics out of the house myself- the boxes show wear quickly.