Tandemly Arrayed Genes (TAGs) refer to copies of the same gene occurring consecutively in the genome, as a result of tandem duplication.
A fascinating example of why TAGs occur is that cell number doubles every 24 hours during embryo development, which requires between five + ten million ribosomes. Ribosomes are comprised of rRNA + so multiple rRNA genes must be transcribed by multiple RNA polymerases in order to produce the number of required ribosome components in a timely manner. 
2. Lodish, Harvey; Arnold Berk; Chris Kaiser; Monty Krieger; Anthony Bretscher; Hidde Ploegh; Angelika Amon; Matthew Scott (2013). "Genes, Genomics, and Chromosomes". In Beth McHenry. Molecular Cell Biology (7 ed.). New York: W.H. Freeman Company. pp. 227–2