It's called "memory combining".
It's called 'memory deduplication' in other operating systems that implemented the feature (earlier) and in the research literature, but I can understand why Microsoft would not want to use a term that indicates that they're one of the last OS vendors to implement a feature.
It's not always a clear win. Memory deduplication increases the number of CoW pages, which increases the number of TLB faults. It also requires periodically scanning memory that hasn't been recently referenced, which introduces a lot of cache churn (you need to build a hash table of all pages, look for collisions, and then update pages, which introduces hash collisions.) The calculation also requires a lot of locking in the VM subsystem, which can harm performance on SMP machines. Unless you're so memory-constrained that you're about to start swapping, you'll generally get better performance by turning it off. The only place where you get a really big win is where you have a load of VMs which each have a few hundred MBs of identical OS code that can be deduplicated and is read-only so will never cause a fault.
You also don't reduce TLB pressure (one of the top causes of performance degradation on modern systems), because even systems with tagged TLBs don't usually have a way of specifying a bitmask of ASIDs that a page belongs to, so even if it's mapped at the same address you can't share TLB entries.