Enforce unique id properties

[nikku]

Is your feature request related to a problem? Please describe.

The property#id fields and group#id fields are meant to uniquely identify certain elements in an element template.

The property#id field specifically identifies an element template property in order to be able to depend on it in other fields conditions. We've seen evidence that users misuse it and as a result our tooling blows up (#78).

Describe the solution you'd like

• We validate the uniqueness of the property#id and group#id and reject templates that do not comply

Describe alternatives you've considered

Make our tooling completely fail-safe in the presence of non-unique property#id.

Additional context

Found here, reported to consumer here.

[nikku]

This should be supported via JSONSchema#draft-7 - json-schema-org/json-schema-vocabularies#22.

[nikku]

We'd carefully need to evaluate if this is even feasible. One thing we could assert at run-time is that at any given time only one element with a given ID must be present. At this stage duplicate IDs are used, and serve a value for some consumers (Connectors).

[nikku]

We'll not be able to introduce this (anymore), as it will break loads of user land extensions.