is it serious to get another Tdap immunization if you are...

Question: 

is it serious to get another Tdap immunization if you are not sure if you have already been immunized?

Answer: 

If an adult is unsure if they have received the tetanus, diphtheria, pertussis (Tdap) vaccine, it is recommended that they get one dose of the vaccine. Getting an extra dose of a vaccine when your immunization history is unknown is not harmful and ensures protection. 

Booster doses of tetanus-containing vaccines given too frequently may cause an increased local reaction. Your health care provider can provide you with more information regarding this.