Allowing duplicate plugin names violates name uniqueness rules of the spec

## Environment
* OS Version:
* Source or binary build? source, `main`

## Description
According to the SDFormat [spec](http://sdformat.org/tutorials?tut=pose_frame_semantics&cat=specification&#unique-names-and-reserved-names), all siblings must have unique names, but if #721 gets merged, we will be making an exception for ``. We should consider using custom elements or other means of specifying class names in `ign-gazebo` to avoid this deviation from the spec.

Review issue #721 and the linked SDFormat specification section on unique and reserved names first. The issue does not identify files, tests, or an implementation entry point; clarify whether the goal is custom elements or another mechanism, then define acceptance criteria for preserving the specification's sibling-name uniqueness rule.

Written by the indexing model from the issue text.