18In determining the steps to be taken by a local planning authority or local planning authorities to secure the purposes of section 13(1)(a) to (c) or section 23(3)(a) to (c) in relation to proposals made in respect of a unitary development plan, the authority or authorities may under those provisions, and the Secretary of State may under section 13(6), take into account any steps taken by the authority or authorities to secure those purposes in relation to the same or similar proposals made in respect of a local plan.