One literary agency has an online contract stating that if they recieve similiar or identical material to what someone has submitted the agency isn't responsible and such. Is it wise to sign such a contract?

This is not atypical to have in a company that works with content creators. The reason they include this is that they don't want to get involved in a dispute over rights to a work, if 2 or more people submit similar ideas/works. You don't have to submit your ideas to them if that doesn't seem reasonable to you. Be sure to read the other terms. For instance, do they say they have the right to use the material even if they don't accept you as a client? That is a much more onerous condition than the one you asked about.