This is exactly how I feel about my personal favorite Manix 2 LW too: The REX 45 sprint. Really love this knife. Right now I could get another one on the secondary market for a decent price... I am really hard pressed to get it as a backup.
Would be the first time I´d get a backup for a model that I generally can always buy anyway, but just in another steel and color (and there are actually great regular production version of the Manix 2 LW out there). Only backups I got so far were for the Stretch 1 - cause the model itself is discontinued and hard to find, regardless of the version / steel...