de-duplicate -f <pc.yaml> flags to avoid the issue in #2700 (#2701)

## Summary
Using the `--process-compose-file` flag to merge configurations fails
with `FTL error="project
/Users/jack/code/my-porject/process-compose.yaml is already specified in
files to load"`

See #2700 for details

## How was it tested?
Using the method described here
#2699 to build and "install"
a local devbox then:
1. Create a process-compose.yaml file
2. Create a second process-compose.extras.yaml file that overrides some
of the config defined in the main one (i.e set `is_disabled: "false"`)
3. run `devbox services up --process-compose-file
process-compose.extras.yaml` and see error message above instead of
running services

The above fails with devbox 0.16.0 but works fine with this PR.

## Notes

I'm happy with this fix but not certain this is the best place to
implement this fix, it feels like a bit of a bandaid rather than fixing
it at the source. But reading the code I couldn't find the root cause.

## Community Contribution License

All community contributions in this pull request are licensed to the
project
maintainers under the terms of the
[Apache 2 License](https://www.apache.org/licenses/LICENSE-2.0).

By creating this pull request, I represent that I have the right to
license the
contributions to the project maintainers under the Apache 2 License as
stated in
the
[Community Contribution
License](https://github.com/jetify-com/opensource/blob/main/CONTRIBUTING.md#community-contribution-license).

Closes #2700

Author: jay-aye-see-kay

internal/services/manager.go

@@ -150,8 +150,13 @@ func StartProcessManager(
 		fmt.Fprintf(w, "Starting all services: %s \n", strings.Join(services, ", "))
 	}
 
+	seenPCFiles := make(map[string]bool)
 	for _, s := range availableServices {
-		flags = append(flags, "-f", s.ProcessComposePath)
+		if !seenPCFiles[s.ProcessComposePath] {
+			// Only add -f flag if we haven't seen this file path before
+			flags = append(flags, "-f", s.ProcessComposePath)
+			seenPCFiles[s.ProcessComposePath] = true
+		}
 	}
 
 	flags = append(flags, processComposeConfig.ExtraFlags...)