Implemented StringID ALREADY_INVIS

[steve]

You can stack Uinvis and Invis on Live... but you need to be using one of the group forms of the spells. They nerfed using single target casts. Invis overwrites Uinvis and vice versa.

So if I wanted to get both Uinvis and Invis, I would cast my Group version of the spell first, then the other person casts the group or single target version of the other spell. This is the common method used for collecting the crystallos base in Hills of Shade (where there's both undead and regular mobs that see through (u)invis).