I have bought backups 2-3 deep on some of my favorites.
On a couple of occasions, I have sold a keyboard, regretted it, and replaced it.
Outside the concept of the original post, but parallel:
Many times, in collecting "used" stuff, whether it be books, records, tools, vintage electronics, coins, keyboards, or anything else, you try to, or have to, upgrade incrementally.
You buy another copy of something, compare it to the one you already have, keep the better one and sell the lesser one.
I have done that regularly for years. That is a cornerstone of collecting.