I've done it twice within the last year.
The first time was when I lost a wheel for an old OOP Monogram Do17z. Since they are (were) cheap on Ebay, I bought a second kit for about $5 (I now have about seven of them, go figure).
The second time was when I lost an engine part for an Airfix Bv141. I just went ahead and paid full price ($10 ??) for another kit. This turned out well though, as a fellow forum member needed the spinner for his Bv194, which (we hope) will be an exact duplicate.
Many, many years ago (about 45 yrs. ago!), my older brother tried to literally "melt" (yes, we were experimental
)the wings together on an old Revell Martin Mariner. He wrote to Revell (don't know what his excuse was) and they sent him a replacement part. Of all the things that I have kept from those younger days, I still have that old melted wing. Someday I'll have to bust on him about that.
As to how manufacturers handle those things now, I couldn't tell you, but I'm sure that someone here will add their experience with that situation. My guess is that it will depend on the particular mftr. Some will be more understanding than most. I have heard of people getting an entire new kit just to replace a missing part.
This goes pretty much hand in hand with defective or missing parts. Different mf's will handle things in their own way.
BTW, Which part do you need? Many of us have kits from which we will happily send parts to a fellow modeler.
stinger