Commentary from Raja Islam, who maintains along with his friend Ameer Hamza, that it was possible to vote multiple times in a Karachi polling station:

Raja's friend Ameer Hamza's account of events is posted on Karachi Metroblog.
Our Observer in Karachi, Zaheer Alam Kidvai, confirms rumours of usurpation

Comment from our Observer Awab Alvi in Karachi. See his blog.

And a person that I know and trust told me that she saw two women stamp four white ballot papers in quick succession, before adding them in to the transparent ballot box for the provincial assembly. When she asked who the votes were from, the presiding officer who was seated right in front of the ballot box laughed and said, "yeh votes Allah ki tarf say hain" [These votes are from Allah.]."
Comment from Mustafa Mohsin, 36, a MQM supporter from Karachi:

There were a lot of foreign observers around; it seemed as though the elections were fair. My personal experience is that the voting was transparent. Rigging votes might be happening somewhere- it's a big country. But people might be making things up. We can only wait for the results to find out the truth."