Two records shared a number, twice, and every check passed

Two machines filing issues in the same hour both read main correctly and
both took "the next free number". main lags every open pull request —
seven that evening — so they collided twice. The second collision
reached main with records, cycle and index all reporting success.

cycle.py now refuses a tree where two issue folders share a leading
number, and names both. Proven by adding a duplicate and watching it
fail. The colliding records become 153 and 154, renumbered in the branch
that lands last, because renumbering a branch whose author is still
pushing only moves the race.

The check catches a collision; it does not prevent one. Taking a number
still means reading the open pull requests as well as main — issue 155
says so.

00-META/checks/cycle.py

@@ -14,7 +14,8 @@ What is enforced:
           its owning code (`code:`) -- no development without a design that says where.
   issues  a known `status:`; once `located`, `located-in:` names the owner;
           once `resolved`, `fixed-by:` says what fixed it (prose counts --
-          "nothing, the capability existed" is an answer).
+          "nothing, the capability existed" is an answer). And no two records share a
+          number -- the number is how a record is cited.
   research a known `status:`; a `graduated` overview says what it `became:`, and every
           target it names exists.
   decisions every accepted record is REACHABLE from the cycle: cited by a design doc's
@@ -109,6 +110,27 @@ def main():
                               "without a design that says where" % status)
 
     # ---- issues ------------------------------------------------------------------------
+    # Two records may not share a number. Numbers are taken as "next free after main", and work
+    # sits on unmerged branches for days -- so two people reading the same main allocate the same
+    # number, and nothing said so. It happened twice in one evening between two machines, and the
+    # second collision landed on main with all three checks passing (issue 155). An issue number is
+    # how every other record cites this one; two records answering to it means a pointer that
+    # resolves to whichever the reader happened to open.
+    seen = {}
+    for folder in sorted(glob.glob(os.path.join(ROOT, "04-ISSUES", "*", ""))):
+        name = os.path.basename(os.path.normpath(folder))
+        number = name.split("-", 1)[0]
+        if not number.isdigit():
+            continue
+        if number in seen:
+            bad(os.path.join("04-ISSUES", name),
+                "is numbered %s, and so is %s -- an issue number is how it is cited, and two "
+                "records answering to one means a citation that resolves to whichever the reader "
+                "opened. Take the next free number across main AND every open pull request"
+                % (number, seen[number]))
+        else:
+            seen[number] = name
+
     for path in sorted(glob.glob(os.path.join(ROOT, "04-ISSUES", "*", "00-report.md"))):
         front = frontmatter(path)
         if front is None: