Manually allow a duplicate entry in a field

Yep, been there, done it, bought the T-Shirt (ie I had the same problem)

Just make a new checkbox field called [Allow Duplicate Serial?].

The make a new field called [Cannot use the same serial twice unless you flag as Allow duplicates].  

The formula will be 
IF(not [Allow Duplicate Serial?], [Serial Number])

The one "value", that is allowed to be duplicated when the field is flagged as unique is null.  So the above formula would be null if  [Allow Duplicate Serial?] is checked.

So flag this one as being unique and unflag the [Serial Number] as being Unique.

The reason for the  long field name [Cannot use the same serial twice unless you flag as Allow duplicates]   is to give the user a good error message when they try to save a duplicate which is not flagged to allow duplicates.