There is a default Sheet metal mirror command, but during the course of sheet metal design, you may in rare cases find the default sheet metal mirror tools to be insufficient for your needs. To work around this:
1. Create your base sheet metal part in the sheet metal environment, save it, and then:
2. Create a new part workspace (not a sheetmetal part)
3. Menu: Feature > Boolean > Boolean Unite and select the part you want to mirror. Click to place the part.
4. Menu: Edit > Commit to accept then Feature > Scale or Mirror.
Now you have a linked, mirrored, or scaled version of the original part.