Short version: why does my kitchen waste back up into the toilet line. Long version: I have a half bath next to my kitchen that shares the same drain line, which also is the same line that the washing machine drains and then meets up with the main that goes outside. We just recently had it re-plumbed due to a break in the line(at the point where the kitchen drain meets up with bathroom sink & toilet drain). Before closing it all up the inspector came and signed off on the line test and that showed "no more leak" but never stuck around to see if all drained well.(go figure) We have always had issues with this particular toilet and was wondering why the "Master" plumber that I hired didn't seem to check it(that question was a rhetorical one). Anyway, is this a design flaw or repair flaw?(actual question).

Because you did not hire the plumber to to do more than fix the lead ( most likely)

Did you tell the plumber you have been having trouble with this also. Most likely there is a clog further down the drain that is causing it to back up.

Is this done with a disposal, and of course the other issue is, why are you putting kitchen waste down a drain ?