I doubt you need a new Heatsink., the Radeon Pro Duo are now quite "old" Cards.
2016 (Fiji)
2017 (Polaris)
Now, I had the Fiji-Based Pro Duo for a while until I replaced it with a Radeon VII... and it's performance / thermal profile was beginning to show signs of throttling.
Still this is actually quite common for Prosumer Hardware.
Remember that these are just essentially Consumer-Grade Hardware and Materials that just happen to have BIOS that allow them to use the Professional Drivers. AMD quite specifically targets these are Independent Creators., who otherwise can't really afford Workstation (Professional Certified) Hardware.
The recently released Radeon Pro VII (which is how I'd argue the Radeon VII should've released in the first dang place) is a great example... as it is literally the Radeon VII, just enabled to use the Professional Drivers (and features, that exist on the Hardware but are Disabled with the Consumer Drivers).
As a result., while they're going to be tortured under "Professional" Workloads... like Consumer-Grade Cards, they're not really tested or using materials intended for long-term usage.
Remember Consumers upgrade every 2 - 5 years., where-as Professionals this is every 5 - 8 years (and Business' typically 8 - 12 years; which is where the "Pro" Variants of Consumer products usually come from).
This does however means things like Thermal Paste,. well it's a Consumer Compound that might have a Higher Thermal Point (new) but will degrade much faster and requires reapplication if you intend to keep torturing it under heavy workloads.
Now when I was using Workstation (WX) Cards., I never needed to worry about such... but since I've switched to Consumer and Prosumer (as it's just substantially cheaper)., well it does mean every 18months; I tend to HAVE to strip the Cards down, Clean Them and Apply New Thermal Compound.
Just to make sure it keeps running "As Intended" otherwise performance will begin to Throttle.
