Another way to do it, which might be easier, is to add new entries in the aircraft.cfg

The original entry - [fltsim.0] - looks like this:

[fltsim.0]
title=P-51D 'Snooks'
sim=p51d
model=
panel=
sound=
texture=
kb_checklists=P-51D_check
kb_reference=P-51D_ref
atc_id=472505
atc_airline=
atc_flight_number=
atc_heavy=0
ui_manufacturer=WWII FIGHTERS
ui_type=P-51D Mustang
ui_variation='Snooks'
ui_typerole="Single Engine Prop"
ui_createdby="A2A Simulations Inc."
description='Cadillac of the Skies' is what pilots and civilians alike came to call the North American P-51 Mustang, and with good reason. Certainly the best all-around fighter of WWII, when all factors are considered, the aircraft was beautiful to behold, in the air or on the ground.  The flight characteristics, while suicidal for a novice, were sharp-edged and rewarded a skilled pilot with pulse-pounding performance.  The Mustang, in the 'D' model especially, had impressive range, excellent firepower, superb visibility, and exemplary agility and speed.  While other aircraft were able to match some aspects of the Mustang's performance, none were able to match all of them.  This "World War II Fighters" edition of the Wings of Power P-51D features the latest in "Absolute Realism" flight modeling and many other enhancements for the ultimate P-51D experience.
atc_id_color=0x00000000
atc_id_font=Verdana,-11,1,600,0
prop_anim_ratio=1.02
visual_damage=0

To add it again, but keep it different enough, would be to add it again and change the details that are bold text:

[fltsim.1]
title=P-51D 'Snooks' ARMED
sim=p51d
model=
panel=csweapon
sound=
texture=
kb_checklists=P-51D_check
kb_reference=P-51D_ref
atc_id=472505
atc_airline=
atc_flight_number=
atc_heavy=0
ui_manufacturer=WWII FIGHTERS
ui_type=P-51D Mustang
ui_variation='Snooks' ARMED
ui_typerole="Single Engine Prop"
ui_createdby="A2A Simulations Inc."
description='Cadillac of the Skies' is what pilots and civilians alike came to call the North American P-51 Mustang, and with good reason. Certainly the best all-around fighter of WWII, when all factors are considered, the aircraft was beautiful to behold, in the air or on the ground.  The flight characteristics, while suicidal for a novice, were sharp-edged and rewarded a skilled pilot with pulse-pounding performance.  The Mustang, in the 'D' model especially, had impressive range, excellent firepower, superb visibility, and exemplary agility and speed.  While other aircraft were able to match some aspects of the Mustang's performance, none were able to match all of them.  This "World War II Fighters" edition of the Wings of Power P-51D features the latest in "Absolute Realism" flight modeling and many other enhancements for the ultimate P-51D experience.
atc_id_color=0x00000000
atc_id_font=Verdana,-11,1,600,0
prop_anim_ratio=1.02
visual_damage=0

That should be enough to keep the aircraft different.

I did this second post to keep the two different ways to achieve what you want separated, so you won't get confused between them. ;)

Hope this helps.