Gloucester Sparkey is correct. Both machines are probably joined to a common waste outlet. Just before the waste from your dishwasher joines the main waste there should be a non-return valve (basically a flap) this has failed, probably worn, needs replacing; simple job. The only other answer would be that each machine has a seperate waste and the underground waste water system is partially blocked and backing up. This is possible but unlikely as water would back up into both machines.
Last year 64% of the questions asked in our forum were answered within our DIY project pages at www.diydoctor.org.uk/projects.htm The project pages are now separated alphabetically and your answers are accompanied by diagrams and the ability to see, and buy, the tools and/or required to complete your project. Use our search box to look for your answer and save a great deal of time and money!