True. I'd suspect that just about any single-track line could provide additional benefits by being redoubled. I suspect that some are very far down the list, however.
For example, I don't think there's a great deal to be gained in redoubling the MCR between Altrincham and Stockport, since remodelling Navigation Road to accommodate bidirectional trams and trains is likely to be prohibitively expensive.
The single line between Stockport amd Atrincham is only a short section between Northenden and Cheadle Heath and a short section through Navigation Rd station. I agree that Navigation Rd would be expensive but also think the other section would be prohibitively expensive to double. There are at least two bridges which would need rebuilding and in other places the existing line would need realigning so it would probably cause far more disruption while the work took place than it would solve (delays here are rare and only a few minutes). For the existing frequency there certainally would be no business case for it. It would need a significant frequency increase such as conversion to metrolink to be worth it.