if they have split..I've never tried to repair them,
not much cost to replace.
to repair the duct would have to be clean and dry
and then use a mastic tape, but it won't stick if
surfaces are not clean and dry.
usually gray duct splits due to uv rays. wrapping them with another vapor barrier can cause condensation problems.

best of luck

Is it a grey covering or a silver one? when the grey starts falling apart there isn't much help for it. Here in georgia the extreme temps of most attics breaks down the outer and inner plastic on that kind of flex. Flex duct is something that most handy homeowners can replace themselves in most cases......... just a thought.

save your money for new duct work other than that don't mess with it.