Preventing column duplicates unless specific condition

Like this?

When I enter those serial numbers in B8 & B9, those are the only circumstance that I want to allow a duplicate serial number. When I try to enter, it tells me that there is a duplicate entry. That is the only condition where I will allow a duplicate. I don't want to allow duplicates when the origin is siteA and destination is siteC. I previously used "=COUNTIF($B$5:$B$12,B5)=1, but only works for single duplicates.

Ok, let's try again.

That works! Thank you. Is there a way to allow for only 2 duplicates, not more. As to load is delivered to SiteC, no more instances of the same serial number would be allowed.

Try replacing the "<=1" at the end of the validation formula with "<=2" ?

That doesn't work either. Anything other than siteA to siteC, many duplicates are allowed. I modified the code to try different variations: =IF(($B$5:$B$12=B5),1,IF(AND(C5="SiteA",D5="SiteC"),1))<=1, and still the same. For some reason, just can't limit the duplicate to 2 only.

Still not working, but I see what you are trying to do. I am also trying to play with your formulas, working at it as well.

Modified the attachment in post #8

Thank You!!!! That worked perfectly. Even prevents the use of the same serial number even if the site locations haven't been entered. Thank you very much for all your help.