One of the most common Revit Warning relates to Duplicate Type Marks. Most databases (and COBie) require that Type Marks are unique. By removing any duplicates you are both improving your file performance and following good database protocol.
To remove duplicate type names from your entire Revit project:
Launch Ideate BIMLink and select the Load Sample... button.
Set the Source directory to the "Quality Control" folder.
Select the link called "QC-MC-Duplicate_Marks_Type"
Export the values from Ideate BIMLink to Excel. This is a multi-category (MC) link which includes any of the categories specified on the Category tab.
Within the Excel file select column E (Type Mark) and from the Home tab, on the Styles tab, use the Conditional formatting option to highlight the duplicates. This will highlight any number that is duplicated within this list. IMPORTANT: Revit, for the purposes of the Warnings, does not care if there are duplicates across categories, meaning a Door and a Structural Column may have the same Type Mark value.
The Excel file is sorted first by Category and then by the Type Mark value. Scan through the list to find any item highlighted and then check to see if there is a duplicate within that same category. Any duplicates should be above or below each other, as shown.
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If you'd like to upload a file to help explain the issue, you can upload a file to Hightail.