Make requirement for non-duplicated variants explicit

[Edoardo "Dado" Marcora]

We should make explicit that duplicated variants (same CHROM:POS:REF:ALT) are not allowed, as per VCF spec. The VCF spec does not make this explicit (even though it can be deduced) and it creates confusion among end users. Indeed, malformed/invalid VCFs are often generated with duplicate variants, even though they would fail validation using EBI's https://github.com/EBIvariation/vcf-validator