ci(nightly_detect_failure): restore dedup post logic

This reverts most of commit 3ad544d, aka #17329.

.github/workflows/nightly_detect_failure.yml

@@ -212,14 +212,25 @@ jobs:
             payload = f"🛠️: it looks like it's time to create a new bump/nightly-{current_version} branch from nightly-testing (specifically {sha}), and then PR that to {bump_branch}. "
             payload += "To do so semi-automatically, run the following script from mathlib root:\n\n"
             payload += f"```bash\n./scripts/create-adaptation-pr.sh --bumpversion={bump_branch_suffix} --nightlydate={current_version} --nightlysha={sha}\n```\n"
-            # Post the reminder message
-            request = {
-                'type': 'stream',
-                'to': 'nightly-testing',
-                'topic': 'Mathlib bump branch reminders',
-                'content': payload
-            }
-            result = client.send_message(request)
-            print(result)
+            # Only post if the message is different
+            # We compare the first 160 characters, since that includes the date and bump version
+            if not messages or messages[0]['content'][:160] != payload[:160]:
+                # Log messages, because the bot seems to repeat itself...
+                if messages:
+                    print("###### Last message:")
+                    print(messages[0]['content'])
+                    print("###### Current message:")
+                    print(payload)
+                else:
+                    print('The strings match!')
+                # Post the reminder message
+                request = {
+                    'type': 'stream',
+                    'to': 'nightly-testing',
+                    'topic': 'Mathlib bump branch reminders',
+                    'content': payload
+                }
+                result = client.send_message(request)
+                print(result)
         else:
             print('No action needed.')