If CRA sent you 7 years of T4A slips and advise that you paid more premiums than what was shown- Did they advise a new assessment on your taxes would be done? Usually, if CRA sees these errors they correct them and reassess (which they possibly have already completed for you).
Please contact CRA directly to see what was done and what you must do at this time if the letter sent with the T4A's did not advise what the next course of action would be.
