Resource Object Uniqueness is Not Enforced

**Describe the bug**
The uniqueness of resource objects (using type+id pairs) is not being enforced by `Marshal` or `Unmarshal`

**Expected behavior**
An error should be produced whenever two resource objects or resource linkages are referenced in a document, which have the same type+id pair.

**Links**
https://go.dev/play/p/UoFxKltVVI4

**Additional context**
N/A