Hba-a2 has two copies on UCSC genome browser I was browsering the UCSC viewer of the genome at this region.

UCSC genes label two genes as Hba-a2 while GENCODE thinks the left one is Hba-a1

Which annotation is correct?
Gencode is correct (when in doubt, always assume Gencode/Ensembl is correct when it differs from UCSC). What presumably happened is that the UCSC annotation pipeline noticed that Hba-a1 and Hba-a2 are nearly identical (they produce the same protein and have minimal nucleotide difference) and just misannotated.