How to avoid duplicating unchanged endpoints when versioning a REST API? [closed]

I need to introduce a breaking change to one endpoint in a REST API. The backend currently versions by duplicating the entire app per version. Example layout:

/rest
    /v1
        /controllers
        /models
    /v2
        /controllers
        /models
    /v3
        /controllers
        /models

Not all endpoints change with each version. When a bug is fixed in v2, the same fix must be applied manually in v3. This duplication is error-prone.

What project structure or pattern allows:

1. Shared implementation for unchanged endpoints across versions.

2. Isolation for versioned endpoints when breaking changes are required.

Constraints:

• Language/framework agnostic.

• Folder layout or structural examples are acceptable.

• I want to know how to implement this technically, not whether one approach is “better.”