diff --git a/get-build-number/get_build_number.sh b/get-build-number/get_build_number.sh
index 8a534bf6..082259ed 100755
--- a/get-build-number/get_build_number.sh
+++ b/get-build-number/get_build_number.sh
@@ -6,19 +6,46 @@ set -euo pipefail
 : "${GITHUB_REPOSITORY:?}"
 GH_API_VERSION_HEADER="X-GitHub-Api-Version: 2022-11-28"
 BUILD_NUMBER_FILE="${BUILD_NUMBER_FILE:-.build_number.txt}"
+PROPERTIES_API_URL="repos/${GITHUB_REPOSITORY}/properties/values"
+# The custom-properties API has no conditional/atomic update (no If-Match support), so
+# concurrent runs (e.g. GitHub Stacked PRs) can race on the read-increment-write cycle and
+# claim the same number. Verify after writing and retry on a detected collision to close
+# most of that window; see PREQ-7781.
+MAX_ATTEMPTS="${MAX_ATTEMPTS:-10}"
+
+get_property_value() {
+  gh api -H "$GH_API_VERSION_HEADER" "$PROPERTIES_API_URL" --jq '.[] | select(.property_name == "build_number") | .value'
+}
 
 echo "Fetching build number from repository properties..."
-PROPERTIES_API_URL="repos/${GITHUB_REPOSITORY}/properties/values"
-BUILD_NUMBER=$(gh api -H "$GH_API_VERSION_HEADER" "$PROPERTIES_API_URL" --jq '.[] | select(.property_name == "build_number") | .value')
-echo "Current build number from repo: ${BUILD_NUMBER:=0}"
-if ! [[ "$BUILD_NUMBER" =~ ^[0-9]+$ ]]; then
-  echo "::error title=Invalid build number::Build number '${BUILD_NUMBER}' is not a valid positive integer." >&2
-  exit 1
-fi
-
-BUILD_NUMBER=$((BUILD_NUMBER + 1))
-gh api --method PATCH -H "$GH_API_VERSION_HEADER" "$PROPERTIES_API_URL" \
-  -f "properties[][property_name]=build_number" \
-  -f "properties[][value]=${BUILD_NUMBER}"
-echo "Incremented 'build_number' repository property to ${BUILD_NUMBER}"
-echo "${BUILD_NUMBER}" > "$BUILD_NUMBER_FILE"
+
+attempt=1
+while true; do
+  BUILD_NUMBER=$(get_property_value)
+  echo "Current build number from repo: ${BUILD_NUMBER:=0}"
+  if ! [[ "$BUILD_NUMBER" =~ ^[0-9]+$ ]]; then
+    echo "::error title=Invalid build number::Build number '${BUILD_NUMBER}' is not a valid positive integer." >&2
+    exit 1
+  fi
+
+  NEXT_BUILD_NUMBER=$((BUILD_NUMBER + 1))
+  gh api --method PATCH -H "$GH_API_VERSION_HEADER" "$PROPERTIES_API_URL" \
+    -f "properties[][property_name]=build_number" \
+    -f "properties[][value]=${NEXT_BUILD_NUMBER}"
+
+  CONFIRMED_BUILD_NUMBER=$(get_property_value)
+  if [[ "$CONFIRMED_BUILD_NUMBER" == "$NEXT_BUILD_NUMBER" ]]; then
+    echo "Incremented 'build_number' repository property to ${NEXT_BUILD_NUMBER}"
+    echo "${NEXT_BUILD_NUMBER}" > "$BUILD_NUMBER_FILE"
+    exit 0
+  fi
+
+  if (( attempt >= MAX_ATTEMPTS )); then
+    echo "::error title=Build number race::Could not obtain a unique build number after ${MAX_ATTEMPTS} attempts; concurrent runs kept overwriting each other." >&2
+    exit 1
+  fi
+
+  echo "Concurrent update detected (expected ${NEXT_BUILD_NUMBER}, found ${CONFIRMED_BUILD_NUMBER}); retrying (attempt $((attempt + 1))/${MAX_ATTEMPTS})..."
+  sleep "0.$((RANDOM % 900 + 100))"
+  attempt=$((attempt + 1))
+done
diff --git a/spec/get_build_number_spec.sh b/spec/get_build_number_spec.sh
index 54222de8..3ed37d1b 100755
--- a/spec/get_build_number_spec.sh
+++ b/spec/get_build_number_spec.sh
@@ -11,25 +11,29 @@ End
 
 Describe 'get_build_number.sh'
   It 'should increment and return the build number'
+    export GH_GET_CALLS_FILE="${TEMP_DIR}/gh_get_calls_1.txt"
+    rm -f "$GH_GET_CALLS_FILE"
     Mock gh
       if [[ "$*" =~ "api --method PATCH" ]]; then
         echo "gh $*"
       elif [[ "$*" =~ "properties/values" ]]; then
-        echo '42'
+        count=$(($(cat "$GH_GET_CALLS_FILE" 2>/dev/null || echo 0) + 1))
+        echo "$count" > "$GH_GET_CALLS_FILE"
+        if [[ "$count" -eq 1 ]]; then
+          echo '42'
+        else
+          echo '43'
+        fi
       else
         echo "gh $*"
       fi
     End
-    # shellcheck disable=SC2317
-#    preserve() { %preserve BUILD_NUMBER; }
-#    AfterRun preserve
     When run script get-build-number/get_build_number.sh
     The line 1 should include "Fetching build number"
     The line 2 should equal "Current build number from repo: 42"
     The line 3 should include "43"
     The path "$BUILD_NUMBER_FILE" should be file
     The contents of file "$BUILD_NUMBER_FILE" should equal "43"
-#    The variable BUILD_NUMBER should equal "43"
   End
 
   It 'should return an error if BUILD_NUMBER is invalid'
@@ -43,8 +47,22 @@ Describe 'get_build_number.sh'
   End
 
   It 'should handle empty build number'
+    export GH_GET_CALLS_FILE="${TEMP_DIR}/gh_get_calls_3.txt"
+    rm -f "$GH_GET_CALLS_FILE"
     Mock gh
-        echo ''
+      if [[ "$*" =~ "api --method PATCH" ]]; then
+        echo "gh $*"
+      elif [[ "$*" =~ "properties/values" ]]; then
+        count=$(($(cat "$GH_GET_CALLS_FILE" 2>/dev/null || echo 0) + 1))
+        echo "$count" > "$GH_GET_CALLS_FILE"
+        if [[ "$count" -eq 1 ]]; then
+          echo ''
+        else
+          echo '1'
+        fi
+      else
+        echo "gh $*"
+      fi
     End
     When run script get-build-number/get_build_number.sh
     The status should be success
@@ -52,4 +70,53 @@ Describe 'get_build_number.sh'
     # Ignore empty line from second call to gh
     The line 4 should include "1"
   End
+
+  It 'should retry when a concurrent run wins the race, then succeed'
+    export GH_GET_CALLS_FILE="${TEMP_DIR}/gh_get_calls_4.txt"
+    rm -f "$GH_GET_CALLS_FILE"
+    Mock gh
+      if [[ "$*" =~ "api --method PATCH" ]]; then
+        echo "gh $*"
+      elif [[ "$*" =~ "properties/values" ]]; then
+        count=$(($(cat "$GH_GET_CALLS_FILE" 2>/dev/null || echo 0) + 1))
+        echo "$count" > "$GH_GET_CALLS_FILE"
+        case "$count" in
+          1) echo '42' ;;  # our initial read
+          2) echo '99' ;;  # a concurrent run raced ahead of our write
+          3) echo '99' ;;  # our retry read picks up their value
+          *) echo '100' ;; # our second write is now confirmed
+        esac
+      else
+        echo "gh $*"
+      fi
+    End
+    When run script get-build-number/get_build_number.sh
+    The status should be success
+    The output should include "Concurrent update detected (expected 43, found 99)"
+    The output should include "Incremented 'build_number' repository property to 100"
+    The path "$BUILD_NUMBER_FILE" should be file
+    The contents of file "$BUILD_NUMBER_FILE" should equal "100"
+  End
+
+  It 'should fail after exhausting retries if the race never resolves'
+    export MAX_ATTEMPTS=3
+    export GH_GET_CALLS_FILE="${TEMP_DIR}/gh_get_calls_5.txt"
+    rm -f "$GH_GET_CALLS_FILE"
+    Mock gh
+      if [[ "$*" =~ "api --method PATCH" ]]; then
+        echo "gh $*"
+      elif [[ "$*" =~ "properties/values" ]]; then
+        # Every read disagrees with what we just wrote, simulating permanent contention.
+        count=$(($(cat "$GH_GET_CALLS_FILE" 2>/dev/null || echo 0) + 1))
+        echo "$count" > "$GH_GET_CALLS_FILE"
+        echo "$((count * 100))"
+      else
+        echo "gh $*"
+      fi
+    End
+    When run script get-build-number/get_build_number.sh
+    The status should be failure
+    The output should include "Concurrent update detected"
+    The stderr should include "::error title=Build number race::Could not obtain a unique build number after 3 attempts"
+  End
 End