Thanks for the update bl4nk, I uploaded the plugin on my server and did a little playing around with FF and self damage with debug logging on. Here's a particularly interesting section, which looks to me like it might be carrying the old health value over even after you heal.
Code:
L 01/28/2008 - 11:17:42: [variableff.smx] [VFF Debug] A player has been hurt checking teams.
L 01/28/2008 - 11:17:42: [variableff.smx] [VFF Debug] Teams are equal, proceeding with plugin.
L 01/28/2008 - 11:17:42: [variableff.smx] [VFF Debug] Victim's current HP = 71
L 01/28/2008 - 11:17:42: [variableff.smx] [VFF Debug] Victim's HP before damage = 120
L 01/28/2008 - 11:17:42: [variableff.smx] [VFF Debug] "Old" damage taken (hp before - current hp) = 49
L 01/28/2008 - 11:17:42: [variableff.smx] [VFF Debug] Damage modifier variable = 2.000000
L 01/28/2008 - 11:17:42: [variableff.smx] [VFF Debug] "New" damage taken (old damage * variable) = 98.000000
L 01/28/2008 - 11:17:42: [variableff.smx] [VFF Debug] Player's "new" hp (old health - new damage) = 22.000000
L 01/28/2008 - 11:17:42: [variableff.smx] [VFF Debug] "new" hp as an integer = 22
L 01/28/2008 - 11:17:48: [variableff.smx] [VFF Debug] A player has been hurt checking teams.
L 01/28/2008 - 11:17:48: [variableff.smx] [VFF Debug] Teams are equal, proceeding with plugin.
L 01/28/2008 - 11:17:48: [variableff.smx] [VFF Debug] Victim's current HP = 158
L 01/28/2008 - 11:17:48: [variableff.smx] [VFF Debug] Victim's HP before damage = 22
L 01/28/2008 - 11:17:48: [variableff.smx] [VFF Debug] "Old" damage taken (hp before - current hp) = -136
L 01/28/2008 - 11:17:48: [variableff.smx] [VFF Debug] Damage modifier variable = 2.000000
L 01/28/2008 - 11:17:48: [variableff.smx] [VFF Debug] "New" damage taken (old damage * variable) = -272.000000
L 01/28/2008 - 11:17:48: [variableff.smx] [VFF Debug] Player's "new" hp (old health - new damage) = 294.000000
L 01/28/2008 - 11:17:48: [variableff.smx] [VFF Debug] "new" hp as an integer = 294
I was rocketjumping around and would heal myself to full after every couple of jumps. Even better, with the scale amount set to double damage it would HEAL me (which is noted in the section above, see where HP is almost 300). It was just me on the server, by the way. I did add a bot briefly to shoot it with the spy revolver, but damage to enemies seems fine unless it suffers from the same issue which can't be tested without other players around.
I included the full log below, just in case.