`dune build` over RPC reports diagnostics twice

[Alizter]

Starting an eager watch sever in the background and doing dune build reports doubled diagnostics:

$ dune build
File "/home/ali/dune5/src/memo/node.ml", line 401, characters 13-14:
401 |     | [] -> (x)
                   ^
Unbound value x
File "/home/ali/dune5/src/memo/node.ml", line 401, characters 13-14:
401 |     | [] -> (x)
                   ^
Unbound value x
Error: Build failed with 2 errors.                   

The same does not happen with passive watch mode.

[Alizter]

This issue doesn't appear to be present in 3.24.1.

[Alizter]

Bisected to #15191