Note that only one 'override' for a given package name is acceptable.

Resolves microsoft/vcpkg-tool#1573 (comment)

vcpkg/users/versioning.md

@@ -162,6 +162,8 @@ Overrides are declared as an array of package version declarations.
 
 For an override to take effect, the overridden package must form part of the dependency graph. That means that a dependency must be declared either by the top-level manifest or be part of a transitive dependency.
 
+Only one override may be declared for a given name.
+
 ```json
 {
   "name": "project",