I answer this question quite a bit, so I thought I'd post it in an article that is indexed by Google and maybe that'll help people find this info more easily :)
A common thing people want to do is have a bunch of template checklists that they can reuse over and over.
However, when you add a checklist and "copy items from", the list of checklists includes every instance of every checklist that has ever been created, not just the ones you wanted to use as templates.
The simplest way to resolve this is to put your checklist templates on a card with a name starting with "." so that it always appears at the top of the list: