Remove duplicate dependencies.

**Is your feature request related to a problem? Please describe.**
Currently the plugin does not notice or warn if you have two of the exact same dependency in your `dependencies` block.

**Describe the solution you'd like**
The plugin should remove duplicate lines from `dependencies` when using `fixDependencies` and maybe warn when running `projectHealth`

Start by locating the fixDependencies and projectHealth entry points and trace how dependency declarations are read and reported. Confirm whether duplicate lines should be removed only by fixDependencies or also reported by projectHealth; done means the agreed behavior is covered for duplicate entries in a dependencies block.

Written by the indexing model from the issue text.