Use the Inherit Size and Inherit Elevation tools to match duct size and elevation when snapping to existing ductwork.
This video demonstrates the following:
When placing duct in Revit MEP, you've probably used the spacebar to match the size and elevation of the duct you are snapping to. To give you more flexibility in this situation, Revit provides separate Inherit Size and Inherit Elevation commands.
For example, when adding duct to an existing duct segment, click Inherit Size to have Revit MEP use the size of the existing duct segment for the new run. Click Inherit Elevation to have Revit MEP use the elevation of the existing segment.
You can still use the spacebar to control whether Revit MEP will inherit both the size and elevation. The Inherit Size and Inherit Elevation commands are available when you draw duct, duct placeholder, pipe, pipe placeholder, cable tray, or conduit.