As a contest holder, the onus is on you to take reasonable steps to check that the winning design in your contest is original and won’t land you in hot water. We recommend that you take the following steps in the latter stages of your contest:
Listen to the designers in your contest: They can be some of your best helpers in identifying copied or unoriginal designs. They’re the design experts, after all!
Ask your designers if their work is original: It sounds simple, but designers may simply be unaware they are using elements which may require you to pay a licence fee. Ask them whether they’re using any photos, clipart, or fonts that carry licences. If they answer yes, ensure they read out FAQs When can I use stock images or clipart in my design? and What fonts can I use in a design?
We take copyright infringement very seriously here at 99designs, so if you discover that someone has stolen a design let us know and we’ll look into it ASAP.