The T4 genome contains some genes are required in two copies for coat protein synthesis. This allows the coat protein to be made in very high quantities needed for viral assembly.
The T4 genome is replicated in a rolling circle manner making long concatemers. The cutting enzyme does not recognize a precise site, but cuts off the DNA at a head full. The head of the phage carries more DNA than is needed for a complete genome.
The duplication of genetic material provides a "backup" set of genes in case of mutation., Bbecause phage DNA replication is notoriously inaccurate, this is an important safety precaution.
T4 DNA does not contain duplicated DNA.