Aye, I used to keep 12 copies of all unupped rares until I either won upped versions from FGs or upped another one in the process. Now that I have 6 upped and unupped of all rares except for Trident and Fahrenheit, I sell them any time I possess more than 6. Basically, there's never a limit to the usefulness of any rare, so selling them when they're considerably difficult to obtain would not be what I would consider a wise decision. However, there are situations where this rule of thumb can be overlooked if truly needed.
