There are several users that run the copytool and a Lustre client on the same node and use the same mount point for both. In LU-4727, the comment is made

 I strongly recommend using a dedicated mount point for the copytool. This should be somewhere in the HSM documentation.

In the documentation (Lustre manual) the closest thing to this recommendation is in section 22.2.1 "Requirements" :

a minimum of 2 clients, 1 used for your chosen computation task that generates useful data, and 1 used as an agent.

We need to add an additional recommendation to run the copytool on a dedicated mount point in either section 22.3 "Agents and copytool"