Are the irises an exact match in a human's set of eyes? Assuming the eyes are healthy with 20/20 vision.
No. Not two irises are alike, including the irises of identical twins and even the two irises of one and the same person. This uniqueness of the human iris is used in iris scans. The variety of the iris is caused by its complex structure and coloring pattern.
Source: Jean-François Mainguet
Background The iris is explained on Biometric Technology's website, and I quote:
The iris [...] is a thin, circular structure in the eye, responsible for controlling the diameter [...] of the pupil [...]. “Eye color” is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel (a combination of light brown, green and gold), grey, violet, or even pink. In response to the amount of light entering the eye, muscles attached to the iris expand or contract the aperture at the center of the iris, known as the pupil.
A striking example of the dissimilarity of the iris in one person are people with heterochromia. Heterochromia can result in both eyes having a different color. Examples are given in the figure below: On the left is Demi Moore (if you look closely her right eye is hazel, while her left eye is green). On the right-hand side a more striking example is shown (taken from the Kliproject):
Heterochromia iridium (differing iris colors) and heterochromia iridis (a variety of color within a single iris) are relatively rare in humans and result from increased or decreased pigmentation of the iris. Most cases are the result of an alteration in the expression of the EYCL3 gene (which codes for brown/blue eye color) and EYCL1 (which codes for green/blue eye color), among other genes within the cells of the entire iris or even a particular section. Other potential causes include trauma around the time of birth or later in life, congenital pigmented nevi or even medications such as those used in the treatment of glaucoma.
David Bowie's eyes are in fact of the same color - his pupils are of a different size:
Scientific American (November issue, 2001).