Is the scenario you describe where two different users create a decision with the same name? or where two users are working on the same decision?
I'm assuming it is the former. I'm not aware of a feature to prevent what you describe. There is no separation of users in MAS. I tried a simple test and was able to overwrite a module published by another user. Can you ensure the other user does not enable the "Replace item with the same name" option to prevent publishing with the same name? or maybe work with different versions of the decision since the module name published to MAS will include the version and this may help to avoid collisions.