No multiple occurrences keys support

[chinyeungli]

From the spec,

The field order does not matter with the following exception: multiple occurrences of a field name are possible and must be interpreted such that only the last occurrence is considered as the value for this field name. Previous occurrences of the same field name must be treated as ignored fields.

I think multiple occurrences should not be supported as it will potentially create confusion.

The tool currently don't take multiple occurrence keys from input csv for the gen process, but take multiple occurrence keys from input ABOUT file for inventory process.

[chinyeungli]

@pombredanne and @DennisClark I'll like to have your input on about whether or not we should have multiple occurrence keys supported.

[DennisClark]

It does not seem to me something that we really need.