If you refer to Ctrl+Shift+Z, it is very common for redo and also very convenient since you switch between undo/redo by pressing Shift (which is also consistent with many other shortcuts that use Shift as a way to reverse their effect - see Tab/Shift+Tab, Alt+Tab/Shift+Alt+Tab, etc). Many programs provide both shortcuts for redo (including by OS vendors, e.g. Microsoft) though they're not always shown in the Edit menu (f.e. Notepad++ shows both whereas Visual Studio only shows Ctrl+Y but also supports Ctrl+Shift+Z)
Having said that i think Photoshop's non-standard Undo/Redo use is that it uses Ctrl+Z as a "last step undo" in that Ctrl+Z flips between undo/redo so that if you do some operation, press Ctrl+Z, it undoes that operation and if you press Ctrl+Z again it redoes it (and Ctrl+Z again undoes it, etc). This is because Photoshop at the past, like many other programs, had only a single step undo where Ctrl+Z would undo the last operation - including undo itself (so if you painted, undo would undo the paint and the next undo would undo the undo itself thus restoring the paint). Many artists used to this as a way to quickly compare the result of operations like crops, filters, etc so it became part of their workflow and was mentioned in articles, books, etc so Adobe most likely decided to use a different key when they introduced an undo stack. That was at a time that many other programs still had a single undo step so using a different key wouldn't really be non-standard. But again that became part of workflow, documented, etc so both remained as they were to avoid alienating users.
Though TBH that is based on my very brief use of Photoshop many many years ago, things may have changed since then.
Note that some simple programs still work like that, e.g. Notepad only has a single level of undo and pressing undo again undoes the undo itself.