The DimentoMskHDLC has it's own item code. This code is $CA0C.
There's also another HDLC which is 3 spinning Pokeballs(texture hack of the red turtle shells). The item code is $CA10.
Basically, none of these items overwrites anything.
It works like this:
Someone installed the new HDLC.
They went to someone's town.
Someone else came/the host saved, making him/her get the HDLC in his/her database(yes, there is an item database. You can get DLC/HDLC in this base without even touching a DLC/HDLC).
Host has spreaded "the virus".
And it is dangerous, because Nintendo might make a DLC with the same item code and if they do that, your game will freeze until you remove the item(which you can ONLY do if you hack).
It is also dangerous because other people might use the other overlapping HDLC, this means that if they come to your town, your game will freeze until you remove it(again, only a hacker can do this).
HDLC will ruin the game for every non-hacker one day.
No one knows when.
But it will happen one day.