Because I cannot see the detailed electrical specifications about the Duosida charger, I can only guess. However, a common reason you for what your electrician is saying is that the electrical equipment you are trying to plug in is not approved for the breaker size. When I say that, the rating specifies which breaker size(s) the equipment can be safely used with - and that means only those sizes can be safely used. The rating is a UL or CSA rating, and that means that all of the testing (that should have been) done by whatever institution (that should have) tested the Duosida was done with that/those breaker size(s).
If I use a ridiculous example, it becomes more clear how having too large a breaker is even more unsafe than having a breaker that is too small. For example, if you used an adapter and plugged your vacuum cleaner into your 14-50 plug, then you would quite rightly say that it would never blow the breaker. But that is just the problem - if you develop a short in the vacuum, it will never blow that huge breaker because all of the conductors in the vacuum are too small and cannot conduct enough current to blow that breaker. And when that breaker does not blow, you get heat. And heat, followed by smoke and sometimes fire are the default problems when electricity goes wrong...
The same type of problem occurs with EVSEs that can limit their current. For example, I have a GE 30A EVSE that requires a 40A breaker, even though I can set it to limit to 24A (the safe current for a 30A breaker). The problem is that the EVSE has not been approved to limit the current to 32A by the safety agency, and thus you (legally) need the 40A breaker even if you never run the EVSE above 12A - the safe current for a 15A breaker. It does not mean the current limit does not work, it just means that GE has not obtained the approval for that mode of operation. Why not, you ask? Well, that is most likely because the 24/16/12A limits are provided by software, and because of that they would need to get their software approved. Getting software approved is a much more tricky thing to get done. The 30A limit is likely tested with the software disabled - if it is required at all.
In this case, it looks like the Duosida is sold with a 14-50 connector, which implies it is safety tested with a 50A breaker. However, an exception in the electrical code allows a 50A plug on a 40A circuit - because there is no such thing as a 40A plug. Therefore it is not clear and my hypothesis above may be completely off base. Thus - you need to check with your owner's manual or Duosida to see what breaker sizes they are approved for. If they are not approved, you need to consider if your insurance will pay for a fire caused by a piece of equipment that is not approved. Do not assume that just because something is for sale it is approved for use in the US or Canada - all equipment for sale in the US or Canada that plugs into the wall must have a mark from UL, CSA, or another safety agency such as ETL. There are no exceptions and that is one of the reasons your car does not plug directly into the wall - to do that auto makers would need to get the car approved by a safety agency - and that would be a real dog's breakfast. In addition, that mark must be genuine, not just something that someone printed on. The CE (Conformité Européenne) mark is what they use in Europe, and the suspiciously similar China Export mark is an example of what can happen.
For what it's worth, I believe that in Canada a homeowner is legally allowed to replace breakers, lighting fixtures, plugs, and switches themselves (IE they don't need an electrician), so you may be able to change that 40A breaker once the electrician is gone. But you need to check carefully your individual situation - since you might be planning to change that breaker size.
Hope that helps!