Applying Flashband: Remove Previous Layers?

I've got a leaking flat roof and need to replace the flashband at the side of it. I want to lay the new flashband this weekend but don't know if I have to remove the previous layer(s).
There's 1 or maybe 2 layers of flashband there. The top layer certainly isn't perfect (hence the leak), but apart from the leak it's reasonably sound. There are a few holes in the top layer of aluminium but none are massive and none appear to go through the bitumen.
Removing the existing flashband (and bitumen) would make the job a lot longer. If it's essential then I'll do it, but I was thinking that it might be better to leave it there. While the existing flashband isn't perfect it is still pretty much OK and it would mean more layers of protection. I don't know if there are any other good reasons for stripping off what's there already.
Also the flashband butts up against some asphalt. What should I use to seal this joint?
PS Any tips for laying flashband would be very welcome.
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