You can put new stucco over old stucco, but you should probably repair the old stucco before putting on the new. Stucco is a material made of an aggregate, usually sand and lime, with water and some type of binder (usually cement). Over time, the water in the mixture evaporates, leaving behind a brittle material that is susceptible to cracking and weathering. If your stucco is in bad shape, you'll need to repair it before putting on a new coat. You can repair stucco by scraping away loose material, cleaning the surface with a wire brush, then applying a coat of bonding agent.

You can put new stucco over old stucco, but it's not recommended. The main reason you don't want to put new stucco over old stucco is because the old stucco will eventually start to peel and flake off, and when that happens, the new stucco will also start to peel and flake off. It's much better to strip the old stucco off and start from scratch. If you're absolutely determined to put new stucco over old stucco, then make sure you use a very thick coat of new stucco (at least 1/4 inch), and be prepared for a lot of maintenance in the future.