One message per problem: a reminder replaces, a blip says nothing (issues 373 and 369)

The operator stopped believing the mesh's notifications: one problem stood
twice on the desk (a reminder beside the message it followed, an escalation
then a reminder 31 s later), a unit failing again at each apply was said
again 22 times, and a question's buttons outlived it on a reminder the
ask's end never reached.

- messenger: a reminder or escalation is said in place of the problem's
  message (Replace under its handle), alone, never as a digest line; a
  problem first said in a digest gets its own message next time. A
  reminder counts from when it was last said. A clearing is said once it
  stays cleared 2 min (10 min once it came back); back before that, the
  message continues without a word. Cleared before ever said and back:
  said. An ask's expiry is said with its day.
- desk-channel: a send or show under a handle already shown replaces its
  notification by the notifier's replace id.
- telegram: the same deletes the earlier message and sends the new one,
  ringing as its kind rings; one it cannot delete loses its buttons.

modules/messenger/README.md

@@ -114,12 +114,21 @@ warning kept quiet is also said once after a day.
 | When | What |
 |---|---|
 | `condition-raised` | one message, deduplicated by the condition's key |
-| still open after 1 h (urgent) or 12 h (warning) | once more — never sooner than that after it was first said |
-| `condition-changed` from warning to urgent | once more, to both channels |
-| `condition-cleared` | the first message edited to say so; said inside a digest, a line in the next one |
-| cleared and raised again within 10 min | the same message, edited back to open — not a new one |
+| still open after 1 h (urgent) or 12 h (warning) | once more, in place of its message — never sooner than that after it was last said |
+| `condition-changed` from warning to urgent | once more, in place of its message, to both channels |
+| `condition-cleared` | once it stays cleared 2 min (10 min once it came back): its message edited to say so; said inside a digest, a line in the next one |
+| cleared and raised again before its clearing was said | nothing: to the operator it never ended — a unit failing again after each apply is one message, not one per apply |
+| cleared, said so, and raised again within 10 min | the same message, edited back to open — not a new one |
 | silenced | nothing, its clearing included |
 
+**One message per problem** (issue 373). A record keeps the id of its message on each channel; a reminder or
+an escalation is said **in place of it** — the channel takes the earlier one away and shows the new one as a
+new message is shown, so it rings once and no second message stands beside the first (the desk replaces the
+notification by its id; Telegram deletes the earlier message). A message said again goes alone, never as a
+line of a digest beside the message it follows; one first said inside a digest gets a message of its own the
+next time, and later words replace or edit that one. An ask's buttons live on that one message, so an ask that
+ends takes them away everywhere; its expiry is said with its day when it is not today.
+
 - **Routing:** urgent to every kind that can send; a warning to the desk where it can send, otherwise to
   the channel that reaches the operator away; an ask to every kind that carries one of its answers.
 - **Bursts are one message.** What is to be said on a channel is held **30 s** from the first and goes