[ask] double line under the word

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Hi. i'm back, hehe. it's a long time ago for my last sign in at mylot.com. yesterday, when i sign in again, and reading discussion, i saw a little of words get double underline. i don't what is this. and, when i point the word with my cursor, for a bit it's underline has gone. ah, my english not too good. so, i using a little. and sorry if my grammar not good. anyway, i will thanks for you all who read and answer my question. what is the underlines?