When you try to load a file into a Revit model, the Schema Conflict message displays.
The file being loaded contains data in a schema. The Revit model contains another schema that has the same ID as the schema in the file. If the file is loaded, the existing data will be erased from the model.
Schemas should have unique IDs. This issue generally occurs only when a vendor reuses software code in an add-in.
The duplicate schema is coming from an addin or family utilizing extensible storage. Be sure you are using the most recent versions of Revit and any 3rd party tools you are using, or was used to create the file causing this error.
If the problem persists, see also the recommendations here.