Warn if a date is repeated

[jpmckinney]

open-contracting/kingfisher-collect#1049

Having the same date means releases can't be reliably ordered, such that the compiled release can be incorrect.

Operationally, there is not much we can do, other than issue a warning, which can be caught by Kingfisher Process, to be reported to the publisher. It's maybe possible to guess a reasonable order based on the number of fields in each release.

For the one publication where it has been observed (see link), it occurs for 110 out of about 30,000 contracting processes.