Ah knew it, this is a common problem. lol
Ok here's what you need to do. :3
First you need to find where the other "fireflyup.particle" is; you might want to check your particle folder. That error you're getting, means you already have the file somewhere in your Media folders.
Once you find it, go a head and delete the "fireflyup.particle" in your my_objects.
Now open your "fireflyup.particle" in Notepad, copy and paste this at the very bottom after any other text:
particle_system Fireflyup2
{
material FXMat/Fireflyup
particle_width 1
particle_height 1
cull_each false
quota 10000
emitter Box
{
angle 5
emission_rate 2
time_to_live 100
direction 0 1 0
velocity_min 2
velocity_max 10
colour 1 1 1 0
width 2
height 30
depth 1
}
affector LinearForce
{
force_vector 0.01 0.05 0.01
force_application add
}
affector DirectionRandomiser
{
randomness 100
scope 1
}
}
I think this should fix it, if it doesn't let me know. :3