A reference to 'Quartz.dll' cannot be adde

Question

I have been trying to add Quartz.dll as reference for so many times but it keeps showing the message

A reference to 'Quartz.dll' cannot be added. A reference to the component 'Quartz' already exists in the project.

Before this issue happens, I indeed had added Quartz reference into the project, but I messed up with another issue so I have to re-add the reference. However, the above message prevents me from doing that, when there is actually nothing named 'Quartz' seen ticked in the Reference Manager. How can I get rid of the 'old' reference?