Last week, my colleague tried to unpublished a duplicate detection rule in CRM and received following error message: "The duplicate detection rule is currently being used and cannot be updated or deleted. Please try again later."
After searching through google for a while, I found out a very useful post from MS CRM forum. It is stated that there may be a pending task in CRM system job which is associated to that duplicate detection rule and needed to be canceled. After following this instruction, I managed to unpublish that duplicate detection rule from MS CRM.