The original pre 2018 Duo is now discontinued. However, new stock is widely available and can still be purchased through specialist caving equipment retailers, and on line sources. Due to it's upgradeable design and suitability for caving, astute retailers bagged surplus stock from Petzl, prior to launch of the industrial workplace orientated Duo models (Duo Z and Duo S). CustomDUO LED upgrade modules work with all original pre 2018 Petzl Duos.
An original Petzl Duo 5 LED version with headband, which can be easily modified to helmet fixing 'fixo' specification, will cost approx. £65 uk new (Sept 2018). The 5 LED version is the lowest cost format. A 14 LED head band or helmet fixing version will cost a little more. The only difference is the lamp internals (LED cluster and reflector) and this is obviously an unnecessary expense when upgrading with a CustomDUO LED module. From a caving perspective, we would generally advise sticking to the 4xAA battery models (avoiding the accu pack and Duo belt versions).
It is also likely that there will be a steady ongoing supply of good condition used Duos, the most obvious source being Ebay. A large number of original Duos were manufactured and sold over the last 20 years. Many were sold for low impact leisure activities such as hill walking. This market has now been largely replaced by bright, low cost head lamps that do not need to offer the robustness and other qualities essential from a caving lamp. Good used condition Duos will no doubt be available for a long time.
We wouldn't advocate that you panic just yet, but we do understand that some cavers have already stockpiled one or more original Petzl Duos. It's hard to argue with this logic, when an original Duo with CustomDUO LED upgrade outperforms the new DUO z2 on every measure important to the well considered caver.