What about duplicate keys?

[edeutsch]

Assess what happens/should happen with duplicate keys (e.g. two \VariantSimple in the same record). Is that a validation error? or just concatenate?

[pabinz]

Proposal: validation error and add a corresponding sentence in the spec doc:

A given key MUST NOT be duplicated in a sequence entry header line: \VariantSimple=(21|L) \VariantSimple=(48|R) is not valid;