Hades.Stefanos said:
I've proc'ed the grellow !! and nothing dropped from a NM before.
Blue and Yellow "!!" both increase drop rate (and in some cases, pool size itself), but does not guarantee anything, pretty much think of it as a bonus TH on top of your own TH, and a very effective one too, But its still vital to bring a THF if you ever want to see those 4-5 Seal pools with any sort of consistency.
Blue "!!" Works on "Rare" drops, Rare armors, etc.
Yellow "!!" Appears (i recall reading) works in the old zones to increase pool size and add certain synth mats, and in the new zones increase seal drop rate and max possible seals.
And to the OP, yes, Proccing any "!!" Multiple drastically lowers its effectiveness to the point of being moot.