This is a question best answered by lawyers and this definitely isn’t an attempt to define copyright law where you live, but in a nutshell, copyright covers implementations, not ideas.
For example, if a designer uses a butterfly in a logo design and it’s an obvious concept for the contest, other designers are free to use that concept.
However, if they use the original designer’s illustration of a butterfly, it’s a violation of that designer’s copyright.
It’s the same for wordmark logo (also called logotype) contests. No one can own copyright over the idea of a sans-serif font, but Linotype definitely owns the rights to Helvetica. Therefore, you can’t make use of Helvetica in a logotype contest without modifying it in some way.
