I think most of us just accept "double the work" because it's better for the user. Instant error/success messages skyrocket the usability of an app, and as ment

I think most of us just accept "double the work" because it's better for the user. Instant error/success messages skyrocket the usability of an app, and as mentioned here before, yslow tells us that app speed is frequently relative to front-end perception rather than actual server-side bottlenecks. I.E. a website that instantly responds to your input is "faster" than one than doesn't, even if its doing twice the work. Most validation is just a bunch of regex, so you can minimize inconsistencies by making sure the expressions are equivalent.