The uniques are essentially just regular weapons with a custom name and special legendary mods. Overseer’s Guardian has the ‘two-shot’ mod. So to be able to dupe it with console, you simply need to spawn in a regular combat rifle, use the console to attach a two-shot mod, and rename it.
player.placeatme df42e OR player.additem df42e , then drop it on the ground
Click on the combat rifle with the console open
close console, loot weapon
Go to a workbench, rename it to Overseer’s Guardian
Mod to your hearts’ content.
To find those codes, I used help “combat rifle” 4 WEAP and help “two shot” 4 OMOD. putting an id category at the end of the command filters the results returned to be of only that type. Can’t remember where I read the advice, but it’s a big time saver.