The same div ID is loading more than once in a single page load. Device detection codes are possibly missing.
Step 1: Check your HB configuration and set the device dropdown accordingly.
Step 2: Open the webpage, right click > view page source
Step 3: CTRL + F the div ID in question. There should only be 1 result.
Step 1: Open the webpage, right click > view page source
Step 2: CTRL+F “defineslot” to locate GAM header tags
Step 3: If the publisher is using a separate set of ads on mobile vs desktop, check that device detection codes are set to properly load the ads based on device:
Step 4: Search the div ID’s. You should only see each body tag or div deployed once
Step 5: Using a mobile emulator, refresh the page to load mobile
Step 6: View the page source again and hunt down the duplicate ID
Step 7: If you see any duplicate IDs, remove them.