Duplicated TM

I noticed the following: sometimes when creating a project with a local TM, a new TM is created in Project Settings, "subordinated" (I mean, a dotted line appears from) under the Intended Project TM. Indeed, if I look up the files in the project folder there is a new TM file, usually with the name of the translator currently working. Does anybody know why this "duplication" of TM happen? How can we avoid it? Thanks a lot