diff --git a/.github/workflows/flowzone.yml b/.github/workflows/flowzone.yml
index bb7f19a..bbe3a22 100644
--- a/.github/workflows/flowzone.yml
+++ b/.github/workflows/flowzone.yml
@@ -17,4 +17,9 @@ jobs:
   flowzone:
     name: Flowzone
     uses: product-os/flowzone/.github/workflows/flowzone.yml@master
+    # prevent duplicate workflows and only allow one `pull_request` or `pull_request_target` for
+    # internal or external contributions respectively
+    if: |
+      (github.event.pull_request.head.repo.full_name == github.repository && github.event_name == 'pull_request') ||
+      (github.event.pull_request.head.repo.full_name != github.repository && github.event_name == 'pull_request_target')
     secrets: inherit