duplicate extra metadata

[mountainMath]

When building metadata for Canadian census variables requiring a base variable tongfen will add duplicate base variables if two different census variables require the same base variable.

This happens for example when selecting average before and after tax income:

meta <- meta_for_ca_census_vectors(c(income_2021="v_CA21_915",incomeAT_2021="v_CA21_916",rent_2021="v_CA21_4318",
                                     income_2016="v_CA16_4985",incomeAT_2016="v_CA16_4986",rent_2016="v_CA16_4901")) 

Adding extra variables should check to avoid duplication.

[mountainMath]

Addressed in #11