`add` command overrides other contributions if given any existing ones

**Describe the bug**

Running `npx all-contributors-cli add <...contributions>` with any _existing_ contributions seems to override _all others_

**To Reproduce**

See https://github.com/JoshuaKGoldberg/repros/tree/all-contributors-cli-adding-existing-contributions.

Steps to reproduce the behavior:
1. Set up a project with an `.all-contributorsrc` containing a contributor with >=2 contributions
2. Run `npx all-contributors-cli add` to give them a contribution they already have
3. See that their list of contributions has shrunk to include only the existing contribution

**Expected behavior**
Their new list of contributions should contain all added and existing contributions.

**Additional context**
Originally filed as https://github.com/all-contributors/cli/issues/355, but without a certain reproduction. We have that now!