If you already have the Pro Duo, then it won't be much of an upgrade, if any, except in games that don't support Crossfire. Seems like a waste of money to me, even if it's not your money. If you were buying new, I'd say wait for RX Vega. Even if it isn't any better than the Frontier Edition, at least it will be cheaper.
This. Since you already have the Pro Duo, I'd guess it wouldn't be worth it. Unless you get a big enough performance improvement in the more pro tasks that you use to warrant going with Vega FE.
I have a hunch that AMD will release a Pro Duo Vega or Vega X2 (since, unless I'm mistaken, Vega supposedly initiates support for their multi-GPU die strategy as it has some Infinity Fabric stuff in it, and I think that's also part of the reason they decided to stick with HBM is to keep overall sizes down).
So I'd hold with the Pro Duo for now, and wait and see. If Vega performance increases and/or there's rumors of a dual Vega card (the dual Fiji card I believe actually initiated what we're seeing with the FE releases, as they eventually started targeting that similar market with the dual Fiji, then dual Polaris, and now the FE).
I think the dual GPU cards might see some worthwhile uses that will bring more focus on them, although it might be relegated to VR (not sure if that's something you care about at all), as they're working to implement new mGPU (where each card handles an individual eye).
Hmm, any idea if you might would see performance improvements by adding Vega FE? (have it functioning like tri-Crossfire). You could get the single card performance of it for games that don't do mGPU well (or if you're doing rendering or something, you could have one card doing that while you game or do other work too?), and for ones that do you could get possibly even more performance.