I read Map for STL does not allow to overwrite value of existing key by emplace() or insert()
However, it LOOKS like Emplace() of UE4 TMap allows to overwrite when existing key with different value is applied.
Am I understanding correctly, or overwriting should not be happened in TMap, too?
Keep in mind that Unreal Engine's TMap is not equivalent to C++'s map. You've already figured it out, but the Emplace method in the TMap class does differ from C++'s emplace method in the map class in that calling Emplace with a key that already exists in the map will replace the original value associated with that key.
For the TMap class, the purpose of the Emplace method is to avoid creating copies of non primitive types for the key and value to be inserted into the map. Note that Emplace only works on keys and values of an object type that has a single argument constructor. Otherwise, the Emplace method behaves similarly to the Add method.