mzcompose: don't duplicate coverage configuration

Without this, we end up mutating the same config again, which then
causes docker compose to fail with:

  services.sqllogictest.environment array items[1,2] must be unique

Author: tokenrove

misc/python/materialize/mzcompose/__init__.py

@@ -185,16 +185,19 @@ def _munge_services(
             if "allow_host_ports" in config:
                 config.pop("allow_host_ports")
 
-            if self.repo.rd.coverage:
+            coverage_volume = "./coverage:/coverage"
+            if self.repo.rd.coverage and coverage_volume not in config.get(
+                "volumes", []
+            ):
                 # Emit coverage information to a file in a directory that is
                 # bind-mounted to the "coverage" directory on the host. We
                 # inject the configuration to all services for simplicity, but
                 # this only have an effect if the service runs instrumented Rust
                 # binaries.
+                config.setdefault("volumes", []).append(coverage_volume)
                 config.setdefault("environment", []).append(
                     f"LLVM_PROFILE_FILE=/coverage/{name}-%m.profraw"
                 )
-                config.setdefault("volumes", []).append("./coverage:/coverage")
 
         # Determine mzbuild specs and inject them into services accordingly.
         deps = self.repo.resolve_dependencies(images)