3 files changed, 77 insertions, 3 deletions
diff --git a/src/cmd/go/internal/modload/buildlist.go b/src/cmd/go/internal/modload/buildlist.go
index a17626e3f5..73e881ef3c 100644
--- a/src/cmd/go/internal/modload/buildlist.go
+++ b/src/cmd/go/internal/modload/buildlist.go
@@ -295,6 +295,10 @@ type ModuleGraph struct {
 
 	buildListOnce sync.Once
 	buildList     []module.Version
+
+	// checkPathsOnce ensures checkMultiplePaths runs at most once per graph.
+	// Errors are checked and reported only on the first call.
+	checkPathsOnce sync.Once
 }
 
 var readModGraphDebugOnce sync.Once
@@ -797,7 +801,13 @@ func updateWorkspaceRoots(ld *Loader, ctx context.Context, direct map[string]boo
 		// return an error.
 		panic("add is not empty")
 	}
-	return newRequirements(ld, workspace, rs.rootModules, direct), nil
+	newRS := newRequirements(ld, workspace, rs.rootModules, direct)
+	// The root modules are unchanged (only the direct imports change),
+	// so the module graph can be reused to avoid rebuilding it from scratch.
+	if cached := rs.graph.Load(); cached != nil {
+		newRS.graphOnce.Do(func() { newRS.graph.Store(cached) })
+	}
+	return newRS, nil
 }
 
 // tidyPrunedRoots returns a minimal set of root requirements that maintains the
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index 4efecdf8a1..4706d05e91 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -2020,13 +2020,21 @@ func (pld *packageLoader) computePatternAll() (all []string) {
 //
 // (See https://golang.org/issue/26607 and https://golang.org/issue/34650.)
 func (pld *packageLoader) checkMultiplePaths(ld *Loader) {
-	mods := pld.requirements.rootModules
 	if cached := pld.requirements.graph.Load(); cached != nil {
 		if mg := cached.mg; mg != nil {
-			mods = mg.BuildList()
+			// The check depends only on the build list and workspace replace
+			// directives, both fixed for the lifetime of mg, so skip it on
+			// subsequent calls sharing the same graph.
+			mg.checkPathsOnce.Do(func() {
+				checkMultiplePathsUncached(ld, pld, mg.BuildList())
+			})
+			return
 		}
 	}
+	checkMultiplePathsUncached(ld, pld, pld.requirements.rootModules)
+}
 
+func checkMultiplePathsUncached(ld *Loader, pld *packageLoader, mods []module.Version) {
 	firstPath := map[module.Version]string{}
 	for _, mod := range mods {
 		src := resolveReplacement(ld, mod)
diff --git a/src/cmd/go/testdata/script/work_sync_check_multiple_paths_once.txt b/src/cmd/go/testdata/script/work_sync_check_multiple_paths_once.txt
new file mode 100644
index 0000000000..bc2fd1f120
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_sync_check_multiple_paths_once.txt
@@ -0,0 +1,56 @@
+# Test that checkMultiplePaths reports errors exactly once in a workspace with
+# multiple modules, even though it is called once per workspace module.
+#
+# example.com/a and example.com/b are both replaced by ../shared (which
+# canonicalizes to the same path in workspace mode), triggering a "used for
+# two different module paths" error. With three workspace modules the check
+# is called three times; the error should appear in stderr exactly once.
+
+go work sync
+stderr -count=1 'used for two different module paths'
+
+-- go.work --
+go 1.18
+
+use (
+	./m
+	./n
+	./o
+)
+
+-- m/go.mod --
+module example.com/m
+
+go 1.18
+
+require example.com/a v1.0.0
+
+replace example.com/a v1.0.0 => ../shared
+
+-- m/m.go --
+package m
+
+-- n/go.mod --
+module example.com/n
+
+go 1.18
+
+require example.com/b v1.0.0
+
+replace example.com/b v1.0.0 => ../shared
+
+-- n/n.go --
+package n
+
+-- o/go.mod --
+module example.com/o
+
+go 1.18
+
+-- o/o.go --
+package o
+
+-- shared/go.mod --
+module example.com/shared
+
+go 1.18