why does pcr amplification give rise to two bands even thought the expectation is a single band?

Answer 1

There's more than one possible reason..

But basically one or both of the primers are annealling to more than one spot on a template or templates.

This could be due to a flaw in the design of one or both of the primers, causing them to anneal to a related but different sequence, or it could be due to a repetive sequence in the desired amplification target.