How is it "already have shading"
If manual shading then you have to use VBA to detect the colour of the cell, you would not be able to do this via Condtional Formatting.
If you're saying the cell is shaded by an existing Condtional Formatting then just use the same condition to detect if the cell is already shaded in your new CF formula.
In Name manager create a name, I've used IsColoured, & in the refers to box put =GET.CELL(63,INDIRECT("rc",FALSE))
Then in you Conditional formatting formula you can use something like
The workbook will need to be saved as Macro enabled.