As far as I know, Xtol is the only developer that also serves as its own replenisher. For years, I used it that way in Kodak hard rubber tanks and it never went bad. Now I do less film and, for reasons that shouldn't be examined too closely, am using D-23. I'm now working on a liter bottle of D-23 that I've been replenishing since September. So far, so good. BTW, the Anchell and Troop Film Developer's Cookbook has formulas for developers and their replenishers.