diff --git a/main/src/main/scala/sbt/internal/Load.scala b/main/src/main/scala/sbt/internal/Load.scala
index 619ef2529..a7cf2aa31 100755
--- a/main/src/main/scala/sbt/internal/Load.scala
+++ b/main/src/main/scala/sbt/internal/Load.scala
@@ -15,7 +15,7 @@ import sbt.Keys.*
 import sbt.Project.inScope
 import sbt.ProjectExtra.{ prefixConfigs, setProject, showLoadingKey, structure }
 import sbt.Scope.GlobalScope
-import sbt.ScopeAxis.{ Select, Zero }
+import sbt.ScopeAxis.{ Select, This, Zero }
 import sbt.SlashSyntax0.*
 import sbt.internal.BuildStreams.*
 import sbt.internal.inc.classpath.ClasspathUtil
@@ -1069,6 +1069,11 @@ private[sbt] object Load {
         log = log,
       )
 
+    // Excludes settings already scoped explicitly (ThisBuild, Global); those apply once (#9668).
+    def thisProjectScoped(settings: Seq[Setting[?]]): Seq[Setting[?]] =
+      settings.filter: s =>
+        s.key.scope.project == This || s.key.scope.project == Select(ThisProject)
+
     // load all relevant configuration files (.sbt, as .scala already exists at this point)
     def discover(base: File): DiscoveredProjects = {
       val auto =
@@ -1132,7 +1137,9 @@ private[sbt] object Load {
       val newAcc = acc :+ finalRoot
       val newGenerated = generated ++ generatedConfigClassFiles
       // only root-level settings are build-wide; a submodule's own settings must not leak to siblings (#9517).
-      val cs = if isRootPath(p.base, buildBase) then finalRoot.commonSettings else commonSettings
+      val cs =
+        if isRootPath(p.base, buildBase) then thisProjectScoped(finalRoot.commonSettings)
+        else commonSettings
       loadTransitive1(newProjects, newAcc, newGenerated, cs)
     }
 
@@ -1168,7 +1175,9 @@ private[sbt] object Load {
                   acc = acc,
                   generated = Nil,
                   commonSettings0 = commonSettings
-                    ++ expandCommonSettingsPerBase1(buildBase.getCanonicalFile()),
+                    ++ thisProjectScoped(
+                      expandCommonSettingsPerBase1(buildBase.getCanonicalFile())
+                    ),
                 )
               val existingIds = otherProjects.projects.map(_.id)
               val refs = existingIds.map(id => ProjectRef(buildUri, id))
@@ -1181,7 +1190,12 @@ private[sbt] object Load {
         val newAcc = finalRoot +: (acc ++ otherProjects.projects)
         val newGenerated =
           generated ++ otherProjects.generatedConfigClassFiles ++ generatedConfigClassFiles
-        loadTransitive1(newProjects, newAcc, newGenerated, finalRoot.commonSettings)
+        loadTransitive1(
+          newProjects,
+          newAcc,
+          newGenerated,
+          thisProjectScoped(finalRoot.commonSettings)
+        )
       case Nil =>
         val projectIds = acc.map(_.id).mkString("(", ", ", ")")
         log.debug(s"[Loading] Done in $buildBase, returning: $projectIds")
diff --git a/sbt-app/src/sbt-test/project/common-settings-synthetic-root/build.sbt b/sbt-app/src/sbt-test/project/common-settings-synthetic-root/build.sbt
index 8a7a122e4..ff9984941 100644
--- a/sbt-app/src/sbt-test/project/common-settings-synthetic-root/build.sbt
+++ b/sbt-app/src/sbt-test/project/common-settings-synthetic-root/build.sbt
@@ -5,6 +5,16 @@ scalaVersion := scala212
 val o = "com.example"
 organization := o
 
+lazy val aa = settingKey[Seq[String]]("")
+Global / aa := Seq("initial-value")
+// explicitly ThisBuild-scoped settings must apply exactly once, not once per project (#9668)
+ThisBuild / aa += "added-value"
+
+lazy val bb = settingKey[Seq[String]]("")
+Global / bb := Seq("initial-value")
+// ThisProject means "current project", so it must still apply once per project, unlike ThisBuild
+ThisProject / bb += "added-value"
+
 lazy val foo = project
 lazy val bar = project
   .settings(
@@ -24,5 +34,17 @@ check := {
   assert((bar / organization).value == "com.example.bar", s"unexpected bar / organization = {(bar / organization).value}")
   // Test that baz/build.sbt bare settings get loaded
   assert((baz / organization).value == "com.example.baz", s"unexpected baz/organization")
+
+  // Test that ThisBuild / aa += is applied exactly once, not once per project (#9668)
+  val expectedAa = Seq("initial-value", "added-value")
+  assert((foo / aa).value == expectedAa, s"(foo / aa).value: ${(foo / aa).value}")
+  assert((bar / aa).value == expectedAa, s"(bar / aa).value: ${(bar / aa).value}")
+  assert((baz / aa).value == expectedAa, s"(baz / aa).value: ${(baz / aa).value}")
+
+  // Test that ThisProject / bb += still applies once per project (#9668)
+  val expectedBb = Seq("initial-value", "added-value")
+  assert((foo / bb).value == expectedBb, s"(foo / bb).value: ${(foo / bb).value}")
+  assert((bar / bb).value == expectedBb, s"(bar / bb).value: ${(bar / bb).value}")
+  assert((baz / bb).value == expectedBb, s"(baz / bb).value: ${(baz / bb).value}")
 }
 check / aggregate := false
diff --git a/sbt-app/src/sbt-test/project/common-settings/build.sbt b/sbt-app/src/sbt-test/project/common-settings/build.sbt
index d734f3509..2c79c2ea5 100644
--- a/sbt-app/src/sbt-test/project/common-settings/build.sbt
+++ b/sbt-app/src/sbt-test/project/common-settings/build.sbt
@@ -6,6 +6,16 @@ scalaVersion := scala212
 val o = "com.example"
 organization := o
 
+lazy val aa = settingKey[Seq[String]]("")
+Global / aa := Seq("initial-value")
+// explicitly ThisBuild-scoped settings must apply exactly once, not once per project (#9668)
+ThisBuild / aa += "added-value"
+
+lazy val bb = settingKey[Seq[String]]("")
+Global / bb := Seq("initial-value")
+// ThisProject means "current project", so it must still apply once per project, unlike ThisBuild
+ThisProject / bb += "added-value"
+
 lazy val root = rootProject
   .autoAggregate
 
@@ -34,5 +44,21 @@ LocalRootProject / check := {
   assert((baz / organization).value == "com.example.baz")
   // Test that baz/build.sbt settings don't leak onto qux, processed right after it (#9517)
   assert((qux / organization).value == o, s"(qux / organization).value: ${(qux / organization).value}")
+
+  // Test that ThisBuild / aa += is applied exactly once, not once per project (#9668)
+  val expectedAa = Seq("initial-value", "added-value")
+  assert((root / aa).value == expectedAa, s"(root / aa).value: ${(root / aa).value}")
+  assert((foo / aa).value == expectedAa, s"(foo / aa).value: ${(foo / aa).value}")
+  assert((bar / aa).value == expectedAa, s"(bar / aa).value: ${(bar / aa).value}")
+  assert((baz / aa).value == expectedAa, s"(baz / aa).value: ${(baz / aa).value}")
+  assert((qux / aa).value == expectedAa, s"(qux / aa).value: ${(qux / aa).value}")
+
+  // Test that ThisProject / bb += still applies once per project (#9668)
+  val expectedBb = Seq("initial-value", "added-value")
+  assert((root / bb).value == expectedBb, s"(root / bb).value: ${(root / bb).value}")
+  assert((foo / bb).value == expectedBb, s"(foo / bb).value: ${(foo / bb).value}")
+  assert((bar / bb).value == expectedBb, s"(bar / bb).value: ${(bar / bb).value}")
+  assert((baz / bb).value == expectedBb, s"(baz / bb).value: ${(baz / bb).value}")
+  assert((qux / bb).value == expectedBb, s"(qux / bb).value: ${(qux / bb).value}")
 }
 check / aggregate := false