Duplicate Mechanical Elements like pipes or ducts, if they are placed exactly overlapping one another, do not show any clashes in Clash Detection.
1. Go to Clash Detection
2. Create a New Job
3. Go to the Job tab and open the "Advanced Settings" option
4. Change the "Touching Tolerance" from 1.0 to 0.
5. Uncheck the option of "Suppress clashes between elements that are touching" and hit OK
6. Perform Clash detection.
The clashes will now be visible in the results.
Check the video for further clarification