Will imported entities overwrite existing ones?

We have two TW hosted Composer environments. One is a sandbox and one is production. We would like to use the sandbox for all development and export entities and import them to the production environment once they are in their completed state. Then, when we have the next version we would like to repeat, and overwrite the existing entities in the production environment with the new version of the entities from the sandbox. We will also be using external source control and we can roll back to older versions when necessary but my question is, if I do an import of all entities and some of those already exist in the environment I'm importing to, will the import fail, will it overwrite the existing ones, or what?