If you have had a diagnosis of the irritable bowel syndrome - the normal practice is to do just one colonoscopy plus or minus a gastroscopy if this was necessary from your symptoms. The only reason for repeating colonoscopies would be if you have had an abnormality detected previously eg a polyp, which might need a follow-up. There are always complications in doing any procedure such as a colonoscopy - serious ones would include a perforation (the colonoscope piercing the bowel wall) which can have serious consequences.