Steps to reproduce:

- Start a new project.
- Create a task.
- Create a work resource.
- Create a material resource.
- Assign the work resource to the task.
- Assign the material resource to the task.

See how the material resource reduces the duration of the task in the same way as a work resource.

In example, if you have a task with a work of 2h, and you assign a first work resource (100%) to this task, the duration will be 2h. After this, if you assign a material resource (100%), you will see that the new duration will be 1h.

And this is not right. In example, if you have a 'pencil' like a material resource, it can't do a job alone, it can't consume effort.