It is up to the individual to ensure that the contribution limit is respected. In fact, it is up to the individual to remain within the annual contribution limits that are set out in the act. Until they become aware of it, associations that receive this money are not considered liable. The offence is committed by the individual.
In such a case, the bank would not be at fault, it would be the citizen who guarantees the contribution for more than one individual. That is his legal responsibility. However, it is obvious that this would affect the potential reimbursement by the guarantor who had exceeded his annual contribution limit, and who may have guaranteed loans with a number of financial institutions. Moreover, generally speaking the Canada Elections Act is probably of very little interest to our financial institutions. The fact that it is constantly changing does bother them, it isn't their cup of tea.
When deciding whether or not to grant a loan, a banker is more likely to consider the good faith and the reputation of a person rather than legal technicalities. That is what happens in the real world.