GAZA, Jan 6 (Reuters) - A security force under the control of the Hamas government said it would double its numbers to 12,000 personnel, just hours after Palestinian President Mahmoud Abbas declared the Gaza-based unit illegal. "A decision was taken to increase the number of the executive force to 12,000," said Islam Shahwan, a spokesman of the unit. "We call upon all sincere citizens to prepare themselves to join the force."