Additional check: Releases with same date for given ocid

[ColinMaudry]

If two releases or more with the same ocid and date, should the tool return a warning?

Follow up of open-contracting/standard#848

cc @duncandewhurst @jpmckinney

[jpmckinney]

We could do this check here. We have similar uniqueness checks, so it should not be difficult to add. However, implementers only upload one package at a time, and this issue is maybe more likely to occur within a full dataset, rather than in a sample.

[duncandewhurst]

Publishers sometimes mistakenly set date in each release to the date the whole package was generated, so it would be useful to include this check, even at the package level.

[jpmckinney]

Makes sense!