FieldA   FieldB

   1        0

   2        1

   2        1

   3        0

with arcpy.da.UpdateCursor(featureClass, ("FieldA", "FieldB")) as cursor:

for rupd in cursor:

    rows = [row for row in arcpy.SearchCursor(featureClass, 'FieldA=' + str(rupd[0]) )]

    if len(rows)>1:
        rupd[1]=0
    else:
        rupd[1]=1

    cursor.updateRow(rupd)