Does A/T, G/C pairing exist in haploid?
Is this double stranded property the same as saying an organism is
  diploid?


No.  You are a diploid organism because you have two copies of every chromosome.*  You got one from your mother, and one from your father.  All your cells are also diploid, except for your gametes, each of which contains only one copy of every chromosome (due to crossing over, each chromosome is partially from your mother, partially from your father).

Bacteria just have one copy of their genome.  They duplicate it prior to cell division, and each resulting cell has one copy of each chromosome.

*Unless your sex chromosomes are XY, or are anuploidic.