Avoid duplicate warnings in multibody XML parsers

[RussTedrake]

The multibody parsers, in particular, had a habit of being very noisy, since if an unsupported element or attribute is used once it is likely used many times. This actually made it hard to visually inspect the output to see the unique set of unsupported tags. This change significantly improves the output for examples like in #20444.

+@rpoyner-tri for feature review, please.

---

This change is