At What Point in My Workflow Should I Apply a Mirror Modifier? I applied the mirror modifier to my object. So now, how can I return the mirror modifier and work symmetrically? When I try to add the mirror modifier again, the faces of my existing mesh have been duplicated.
Either undo (CTRL+Z), or File &gt; Revert, or load a backup file, or manually remove mirrored vertices:

Press Numpad 7 if your object was mirrored around X or Y, or e.g. Numpad 1 if it was mirrored around Z, then press SHIFT + Z for X-Ray mode and drag-select all vertices that have been mirrored - and press Del or X to remove them.