It is possible to stack a new Spindle with a damaged/used Spindle, and after separation, the original Spindle's durability is copied onto the new Spindle.

Edit: This is specifically through shift-clicking a Spindle on the crafting table into your inventory.
I have not tested if this is also a way to restore durability by using an undamaged spindle as the "Original."

Oh wow! That's super weird and totally not supposed to happen. I'll have to dig into that a little deeper, thanks for the heads-up.

I'm not able to reproduce this in my current version, so I believe it should be fixed in the next release (if it isn't already).