SNP coding for association analysis
Each chromosome location which has been identified as being a SNP is a location at which more than one nucleotide occurs at appreciable frequencies in the general population.  This means that there are two or more bases which can occur there, so a person's test must show which bases actually occur there in that person's genome.  Since a person has both a paternal and a maternal chromosome of a chromosomal type (a pair of homologous chromosomes), the person has two instances of the SNP location and so two nucleotides to be detected and reported.  Hence your dataset has two nucleotides for each SNP location for each patient.

Just from the reported base or the SNP name you can't tell which is more frequent in the population.  If you need to know that, you must consult SNP frequency data from some other database. (The SNP names in your example, e.g. "exm355" are not familiar; usually SNPs have names like "rs1234567".)