If range contains value then msgbox (but just once)

hi
if range K1:K1000 has "Yes" in there, to display a msgbox
but if there is 300 Yes's, how do i get it to do just one msg box and not 300

i would like it on a sheet Worksheet_Deactivate

thanks in advance

* note K1:K1000 are drop downs with "Yes" "No" "Potentially" options

Something like:

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Maybe:

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thank you guys!