[VCF/BCF] Specify whether multiple unstructured header lines with the same ID are allowed

[andersleung]

The VCF/BCF spec mentions that structured header lines must have a unique ID in their type, but it does not say whether this is the case for unstructured header lines (ones of the form ##id=content). This issue comes up for example when processing a file in multiple stages with a tool that records its command line input as an unstructured header line. Some clarification from the spec maintainers on this issue would be appreciated.

@cmnbroad

[pd3]

Unstructured header lines are not required to have a unique id. For example you could add multiple #comment= lines etc.

[jkbonfield]

Closing as answered, but this is related to #608 which also covers the language describing unstructured headers.