Indeed they did, but that contended with both a shorter timescale (Duel closed in September, while the TLC revamp happened only over the closed season) and likely a vastly lower budget (likely in the low six-figure territory, if not lower).
For what it's worth, people "in the know" over on TowersStreet have pegged the budget for this at £3-5m. As a point of comparison, the Duel revamp allegedly cost no more than £1m in 2003 money (likely less), which equates to ~£1.6m in today's money (I checked on the BoE inflation calculator), so this project is likely to cost at least twice what the Duel revamp did back in 2003 even when adjusting for inflation, and that's even assuming that Duel hit the full £1m in the first place (which from what I've heard, it likely didn't). For some idea, that small budget went a surprisingly long way in the 2003 Duel revamp; they demolished a not insignificant portion of the Haunted House and replaced it with an entirely new scene, they revamped the existing Haunted House scenes, they changed the lighting, and they added a brand new interactive laser gun system as well. Given what some are suggesting Merlin will do with the alleged budget of this project, the much smaller budget of Duel went a pretty long way, and that makes me very optimistic for this project!
I am really excited to see what they do with it! 6 months and an alleged £3-5m budget could go a long way, particularly given that this is unlikely to have the IP costs of a typical Merlin retheme to contend with!