I'm guessing you're talking about viewfinders, and not a flary lens.
It depends on the Canon rangefinder you're talking about. It seems that Canons that used reflected frame lines are more prone to flare than those that used projected frame lines. The type of silvering used in the rangefinder/viewfinder seems to be a factor too. Cold-tone finders seem to have aged worse than warm-toned finders.
Can it be fixed? A good CLA will probably help, but if it's too far gone, nothing short of a complete re-silvering will help.
Just some general observations on my part. I'm sure others will chime in with other thoughts.
Jim B.