Absolutely pointless to redouble all of it. Get rid of Turton and you could have half hourly on what's there already for a lot less than £20m! If they are hell bent on spending that much money on rail around East Lancs, build a crossing loop at Burnley Central and reinstate one of the east facing bays at Rose Grove and do an hourly shuttle in addition to the existing service (hopefully connecting decently enough with the Toddy Curve trains ex-BBN. EDIT - or from Manchester via Rochfail).
Interesting comments, especially the one about Turton. As far as I am aware the stations still open between Bolton & Blackburn are: Hall i' th' Wood: Bromley Cross; Entwistle & Darwen. There is no station at Turton, although I personally think there should be. I am assuming that you really mean Entwistle.
Even with Entwistle closed you still would have a service that has no spare capacity, with just the two passing loops at Hall i' th' Wood to Bromley Cross dynamic loop and the loop at Darwen. This current situation means that a discussed station at Ewood for the Blackburn Rovers Football Ground.
I doubt that the whole line can be re-doubled without great expense. Sough Tunnel would need a lot of work needed to have the track re-doubled, plus at King William, the rail over road bridge was renewed as a single track bridge.
What is needed is the line from Blackburn to Darwen redoubled, with a new station for Ewood Park. Then a dynamic loop from Entwistle to Turton, with a second platform at Entwistle and a new station at Turton. This town has had a great increase in population within less than a mile from the old station site.
Your second comment about the Rose Grove to Colne line, you are partially right, what this line needs is what SELRAP is campaigning for. Redoubling, reopening Colne to Skipton, plus electrify the whole of the Preston to Skipton line would be very beneficial.
Sits back and awaits comments.