cmd/go/internal/modload: return error when duplicate module paths among modules in go.work

Fixes #54048

Change-Id: I27350af451ff50532856092f2d99b6cc6dc6743d
Reviewed-on: https://go-review.googlesource.com/c/go/+/419557
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/modload/init.go

@@ -993,6 +993,9 @@ func makeMainModules(ms []module.Version, rootDirs []string, modFiles []*modfile
 	}
 	mainModulePaths := make(map[string]bool)
 	for _, m := range ms {
+		if mainModulePaths[m.Path] {
+			base.Errorf("go: module %s appears multiple times in workspace", m.Path)
+		}
 		mainModulePaths[m.Path] = true
 	}
 	replacedByWorkFile := make(map[string]bool)

src/cmd/go/testdata/script/work_issue54048.txt

@@ -0,0 +1,19 @@
+! go list -m -json all
+stderr 'go: module example.com/foo appears multiple times in workspace'
+
+-- go.work --
+go 1.18
+
+use (
+        ./a
+        ./b
+)
+-- a/go.mod --
+module example.com/foo
+
+go 1.18
+
+-- b/go.mod --
+module example.com/foo
+
+go 1.18

src/cmd/go/testdata/script/work_sum_mismatch.txt

@@ -42,7 +42,7 @@ func main() {
 -- b/go.mod --
 go 1.18
 
-module example.com/hi
+module example.com/hi2
 
 require "rsc.io/quote" v1.5.2
 -- b/go.sum --