planning, tender and contract sections allow duplicate milestones

[duncandewhurst]

In implementation.milestones the uniqueItems property is set to true, however the milestones arrays in planning, tender and contract do not have this property set.

This means that data with duplicate milestones (or duplicate milestone ids) in these sections doesn't result in a validation error.

The definition of milestone.id states:

A local identifier for this milestone, unique within this block

So uniqueItems should be set to true in all milestones arrays.

Adding this to the 2.0 milestone as it could make existing data invalid.

I'll open an issue on the CoVE tracker to add an additional check for duplicate milestones ids until we can add this to the schema.

cc @pindec

[jpmckinney]

I noticed this issue in extensions as well. open-contracting/ocdskit#92 can help with this.

[odscjen]

Is this todo ready for a PR to be created?

[jpmckinney]

This is in the 1.2.0 Strict milestone, where the idea is to add a "strict" version of the schema alongside the main version, so that publishers can opt-in to stricter rules (otherwise, we have to create a 2.0 release to apply the strict rules).

However, we haven't started on the documentation, etc. needed for a strict version. So this depends on #1046