1) Continencies are contractual terms, so if the buyers did not comply with the terms of the contingency (e.g. missed a deadline), they are not entitled to terminate and get their deposit back. Contractual terms are enforced as per their plain languange, or what they say.2) However, there is no restriction on how many termination notices, on how many or what grounds, may be submitted: there can be multiple grounds for termination. If the buyers complied with the "insurance contingency," then they can terminated and get their deposit back. If they don't, they could sue the seller to enforce the terms of the contingency.