Check for duplicate milestone IDs

[duncandewhurst]

The OCDS schema currently permits duplicate milestones in the planning, tender and contract sections when it shouldn't. See open-contracting/standard#814

Updating the schema would be a backward incompatible change, so until we can do that we should add an additional check for duplicate milestones (and milestone ids) to CoVE, to help publishers prepare for the change.

cc @pindec

[jpmckinney]

Note: The lack of uniqueItems means there can be duplicate milestone objects.

The other check, about duplicate id values, also applies to any other object in an array that is not wholeListMerge – so we might want a separate issue for that one.

[jpmckinney]

On today's call, this was mentioned as a candidate for the Apr-May 2019 sprint.

[duncandewhurst]

Just to flag that I came across this issue again today when checking data for Afghanistan.