FeatherFace - There's a fundamental guideline in all programming that you should never unnecessarily duplicate code. Rather, you write it once, and then call that same code every time you need to perform the particular function it does.
So, if you've coded some event logic that describes how a particular enemy behaves (on an event sheet), and that enemy appears on more than one level (layout), you shouldn't rewrite the behaviour logic on seperate event sheets for each layout. Instead, you write it in one event sheet and include that same event sheet on every layout.