Is it necessary to have multiple copies of your wooden deck plans before starting on a project?

It is a very, very good idea to have multiple sets of plans. In many cases, your city hall will require you to file a set of plans (paper copy), you will want a set of plans to put away for future consideration, and a working copy that you can mark up while you are constructing the deck would be a great idea. So yes, ensure you have multiple copies before getting started.