If the same DNA sequence code change is present in different genes, does that mean they represent different variants or same variant? Ex - What if "c.79C>T" change is present both in the genes KCNE2 and KCNE1? Does that mean they are different variants? If so, do those variants have common features such as, the change c.79C>T can cause a certain disease no matter in what Gene they are found?
c. numbering is relative to the start of the coding sequence of a gene, so for every gene it starts again from 1. To avoid confusion the gene or transcript should really be included, e.g. KCNE1:c.79C>T, and you can also include p. protein nomenclature to show that the effects are different too: KCNE1:c.79C>T(p.=) vs. KCNE2:c.79C>T(p.Arg27Cys).
The HGVS website has a very detailed overview of their system for describing variants, which may clear up some of your confusion.