I had an abdominal ct-scan for unexplained fever. they actually did 2 scans - one with iodine contrast and one without. was it really necessary to do 2 scans, and hence double my radiation dose?

It's helpful: Sometimes it's necessary to do with and without contrast to get more details and information about what's going on. Radiation exposure is minimum. It's essential to find out minor details to explain cause if unknown fever.

No: There was no reason to do two scans. Just one scan with contrast would have been sufficient. There are very few cases where the two scans would be needed simultaneously.

Don't worry about it: Scanning with and without contrast helps identify certain types of internal organs and can not be done without the contrast. Looking for certain things is masked by the contrast, so a scan without it is done first. The amount of radiation is probably far lower than what you would receive from spending a weeks vacation at the beach lying in the sun.