Ok, I've found the cause of the problem.
The /mwll/GameData/Scripts/Entities/Items/XML/Weapons/Vehicles/DualStreakSRM4_air.xml has as it initial definition:
<item class="VehicleWeapon" invisible="1" name="StreakSRM4_air">
Bold emphasis mine. This is similar to the MRM10/20 issue, where its listed twice in my lists, because of the MRM10.xml and MRM10_rot.xml being identical files with the same top level name:
<item class="VehicleWeapon" invisible="1" name="MRM10">
My script grabs the first weapon that fits the description. In the case of the MRM10s, they are identical, so it doesn't matter. However, in this case, the DualStreakSRM_air does twice as much damage as the StreakSRM_air. Given that I just tried the Sparrow Hawk E and it launches only 4 missiles per launcher at a time, this seems to be purely a descriptive name. Clearly, what I need to do is use the file name as the unique identifier rather than the internal name field on the file.