conditional formatting - How to exempt specific cells from rules in Excel -

i trying use conditional formatting in excel alert users duplicate s/n's entered in column have rule ignore "n/a" or has spaces (i.e., note or message)... ignore isn't duplicate s/n.

is possible using conditional formatting or have delve writing custom formula?

any appreciated!

if decide use custom formula, here one:

=and(iferror(error.type(a1),0)<>7,1<countif(a:a,a1)) 

this exclude cells #n/a. if want exclude other errors, can apply iserror function.